backgammon: fix lint warnings

- add or adjust /* FALLTHROUGH */ where appropriate
- add __unreachable() after functions that can return but won't in
this case, and thus can't be marked __dead easily

GC descr and helpm.

Remove ifdefs for V7 (as opposed to older) unix.

Pass the move info around instead of using it as a global.

rename text() to wrtext(), aligning it with most other output routines,
and avoiding a compile error on powerpc64.

XXX: this tries to make a ".text" symbol, which already exists...

sprinkle static

Remove the \n and tabs from the __COPYRIGHT() strings.

convert __attribute__s to applicable cdefs.h macros

KNF and WARNS=3 fixes

Cast last argument of execl to (char *).

Reviewed by <hubertf>.

Mark unused parameters, nonreturning functions and format functions in
the few cases not already marked.

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

Patches provided by Joel Baker in PR 22269, verified by myself.

Negative exit code cleanup: Replace exit(-x) with exit(x).
As seen on tech-userlevel.

remove redundant decl

Use setgid(), not setregid().

Adapt to change of "termcap.h".

As in previously sent patches: backgammon(6) does not need setgid
games privileges from dm, so should revoke them on startup.

Patch submitteed in PR 8024 bye Joseph Myers <jsm28@cam.ac.uk>

The patch below makes the game backgammon(6) use `const' where
appropriate. It also removes some obsolete, unused code to check the
number of users on the system (obsoleted by dm); removes a prototy

The patch below makes the game backgammon(6) use `const' where
appropriate. It also removes some obsolete, unused code to check the
number of users on the system (obsoleted by dm); removes a prototype
for an unused function that was removed earlier; and removes a control
character from one message that was I think intended to clear the
screen on some particular terminal (though I can't find the comment to
that effect in any of the BSD source trees I have handy) but no longer
serves any useful purpose.

From PR 6580 by Joseph Myers <jsm28@cam.ac.uk>.

Also contains two more patches, one in teachgammon/tutor.h (by Joseph)
and one in common_source/fancy.c (by me).

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Fix args processing of backgammon and teachgammon, closes pr 5345 by
David Brownlee.

Use symbolic names for signals(!), open(2)-constants, ...
Fix applied as per PR 6058 by Joseph Myers <jsm28@cam.ac.uk>

KNF & WARNSify, and only compile common_source once/* (compiles 25% faster)

Use POSIX tty semantics.

clean up import

Add RCS identifiers.