Some changes from Michael K. Johnson for Linux.

Some changes from Michael K. Johnson for Linux.
	* sysdep.c (sys_siglist): Don't define this if HAVE_SYS_SIGLIST is
	#defined.  That lets the system provide it, if it has it.
	* syssignal.h (sigmask): Only define this if <signal.h> hasn't
	given us a definition already.
	* syssignal.h (sys_sigpause): Fix argument in prototype.
	* sysdep.c (init_signals): The masks are called empty_mask and
	full_mask, not signal_empty_mask and signal_full_mask.
	(signal_handler_t): Moved ....
	* syssignal.h: ... to here.
	* systty.h (EMACS_SET_TTY_PGRP): Call tcsetpgrp with the correct
	arguments.
	* emacs.c (main): Don't try to establish signal handlers for
	SIGBUS and SIGSYS unless they're actually #defined.