Allow trap to work on ignored signals when the shell is interactive.

Don't hard ignore signals that were ignored by our environment, because
when we try to set a trap on them it will not work. Also while I am here:
1. don't change the action status if the signal syste

Don't hard ignore signals that were ignored by our environment, because
when we try to set a trap on them it will not work. Also while I am here:
1. don't change the action status if the signal system call failed.
2. don't try to sigignore it if signal failed.
3. clear the signal mask in case our parent blocked it for us.

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PR/28940: David Laight: /bin/sh doesn't quote the output of trap.

Use '\0' instead of NULL in two checks (we are not checking for a pointer
value). While here, add a missing whitespace.

Move UCB-licensed code from 4-clause to 3-clause licence.

Patches provided by Joel Baker in PR 22249, verified by myself.

Fixes from David Laight:
- ansification
- format of output of jobs command (etc)
- job identiers %+, %- etc
- $? and $(...)
- correct quoting of output of set, export -p and readonly -p
- differentia

Fixes from David Laight:
- ansification
- format of output of jobs command (etc)
- job identiers %+, %- etc
- $? and $(...)
- correct quoting of output of set, export -p and readonly -p
- differentiation between nornal and 'posix special' builtins
- correct behaviour (posix) for errors on builtins and special builtins
- builtin printf and kill
- set -o debug (if compiled with DEBUG)
- cd src obj (as ksh - too useful to do without)
- unset -e name, remove non-readonly variable from export list.
(so I could unset -e PS1 before running the test shell...)

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VFork()ing shell: From elric@netbsd.org:
Plus my changes:
- walking process group fix in foregrounding a job.
- reset of process group in parent shell if interrupted before the wait.
- move INTON

VFork()ing shell: From elric@netbsd.org:
Plus my changes:
- walking process group fix in foregrounding a job.
- reset of process group in parent shell if interrupted before the wait.
- move INTON lower in the dowait so that the job structure is
consistent.
- error check all setpgid(), tcsetpgrp() calls.
- eliminate unneeded strpgid() call.
- check that we don't belong in the process group before we try to
set it.

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Extended functionality of the trap builtin, which now closely follows
POSIX recommendations.

- trap now accepts signal names and signal numbers
e.g. INT, SIGINT, 2
- added option -l that output

Extended functionality of the trap builtin, which now closely follows
POSIX recommendations.

- trap now accepts signal names and signal numbers
e.g. INT, SIGINT, 2
- added option -l that outputs a list of valid signals
- added signal EXIT to list of valid signals
- a `-' in the action part will reset specified signal to their
default behaviour
- changed standard output format to make it suitable as an input
to another shell that achieves the same trapping results

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remove redundant declarations and nexted externs.

Back out previous vfork changes.

Now we use vfork(2) instead of fork(2) when we can.

Fix bin/9184, bin/9194, bin/9265, bin/9266
Exitcode and negation problems (From Martin Husemann)

When we execute commands from a shell script, make sure that the signals
are being caught (reported by Alexis Rosen), similar to the -c case.

#!/bin/sh
vi "$@"

^C when the script is running...

Fix the -c problem differently. We cannot just ignore SIGINT etc, otherwise
we cannot interrupt sh -c <command>

PR/6213: Urban Boquist: /bin/sh does not handle a trapped signal correctly
The problem was that system calls got restarted after a signal,
instead of returning EINTR. Thus the read builtin, had n

PR/6213: Urban Boquist: /bin/sh does not handle a trapped signal correctly
The problem was that system calls got restarted after a signal,
instead of returning EINTR. Thus the read builtin, had no way to
know that a signal occured that could change the course of execution.
Since the code has sprinkled checks for EINTR all over the place,
it is supposed to work properly with non restartable syscalls.
The fix is to use siginterrupt(signo, 1), before setting a signal
handler, to make sure that system calls don't get restarted.

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Delint.

Fix compiler warnings.

PR/2808: Remove trailing whitespace (from FreeBSD)

Ignore result of sigaction when setting traps. Traps will succeed even
on SIGKILL or SIGSTOP. This is what other bourne shells do. (suggested
by mycroft)

Avoid trapping SIGKILL. Pretend that we did, so that we will not keep
failing trying to trap it later. This is what the other bourne shells do.

Merge in my changes from vangogh, and fix the x=`false`; echo $? == 0
bug.

convert to new RCS id conventions.

be more careful with casts.

clean up further. more patches from Jim Jegers

Add RCS ids.