add section to mmap Xr

Implement MAP_STACK option for mmap(). Synchronous faults (pagefault and
syscall) confirm the stack register points at MAP_STACK memory, otherwise
SIGSEGV is delivered. sigaltstack() and pthread_att

Implement MAP_STACK option for mmap(). Synchronous faults (pagefault and
syscall) confirm the stack register points at MAP_STACK memory, otherwise
SIGSEGV is delivered. sigaltstack() and pthread_attr_setstack() are modified
to create a MAP_STACK sub-region which satisfies alignment requirements.
Observe that MAP_STACK can only be set/cleared by mmap(), which zeroes the
contents of the region -- there is no mprotect() equivalent operation, so
there is no MAP_STACK-adding gadget.
This opportunistic software-emulation of a stack protection bit makes
stack-pivot operations during ROPchain fragile (kind of like removing a
tool from the toolbox).
original discussion with tedu, uvm work by stefan, testing by mortimer
ok kettenis

show more ...

use fancy .In macro for includes. from Jan Klemkow. ok jmc schwarze

Add pthread_attr_[sg]etguardsize() to match rthread, including manpages
Bump lib minor

ok otto@ kurt@ marc@; doc review by jmc@

Add manpage for pthread_attr_[sg]etstack(3)
Correct misnaming in STANDARDS section
ok brad@ otto@

convert to new .Dd format;

Add missing const for function parameters to match POSIX.

From: FreeBSD's libc_r

ok marc@

RETURN VALUE -> RETURN VALUES in .Sh, as per mdoc template.
also new sentence, new line.

copyrights added;

these are all David Leonard, and Public Domain
ok deraadt@

o) start new sentence on a new line;
o) always close .Bl tags;
o) fix usage of .Xr;

millert@ ok

Mostly punctuation fixes.

Trailing whitespace begone!

document