Document abort() bugs.

Document abort() bugs.

* doc/posix-functions/abort.texi (abort): Mention anomalies.
* doc/posix-functions/setjmp.texi (setjmp): Mingw has setjmp.
* doc/posix-functions/sigsetjmp.texi (sigsetjmp): Cygwin has
sigsetjmp.
* doc/posix-functions/siglongjmp.texi (siglongjmp): Cygwin has
siglongjmp, but only as a macro.
* doc/posix-functions/_longjmp.texi (_longjmp): Mention that this
is obsolete.
* doc/posix-functions/_setjmp.texi (_setjmp): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>