Remove dead code dealing with POSIX_SIGNALS.

Remove dead code dealing with POSIX_SIGNALS.
* atimer.c (set_alarm): Remove dead code, all USG systems define
POSIX_SIGNALS.
* data.c (arith_error): Likewise.
* keyboard.c (input_available_signal, handle_user_signal)
(interrupt_signal): Likewise.
* process.c (sigchld_handler): Likewise.
(create_process): Remove if 0 code.  Remove HPUX conditional when
!defined (POSIX_SIGNALS), it cannot be true.
* syssignal.h: Remove USG5_4 and USG conditionals when
!POSIX_SIGNALS, they cannot be true.