Remove POSIX_SIGNALS.

Remove POSIX_SIGNALS.
* s/usg5-4.h (POSIX_SIGNALS):
* s/netbsd.h (POSIX_SIGNALS):
* s/msdos.h (POSIX_SIGNALS):
* s/ms-w32.h (POSIX_SIGNALS):
* s/hpux11.h (POSIX_SIGNALS):
* s/gnu.h (POSIX_SIGNALS):
* s/gnu-linux.h (POSIX_SIGNALS):
* s/freebsd.h (POSIX_SIGNALS):
* s/darwin.h (POSIX_SIGNALS):
* s/cygwin.h (POSIX_SIGNALS):
* s/aix4-2.h (POSIX_SIGNALS): Remove definition.
* s/unixware.h:
* s/sol2-6.h: Remove comments on POSIX_SIGNALS.
* process.c (create_process):
* syssignal.h:
* sysdep.c (wait_for_termination, init_signals):
* process.c (create_process):
* msdos.c: POSIX_SIGNALS is always defined on all platforms,
remove all code that assumes the contrary.