Make sysdeps/generic/internal-signals.h less stubby.

Three of the functions defined by internal-signals.h were not actually
fulfilling their contracts when the sysdeps/generic version of that
file was used.  Also, the Linux version included several more headers
than the generic version, which is the root cause of a build failure
on Hurd (already addressed in another way, but I think it is proper to
make the headers match).

	* sysdeps/generic/internal-signals.h: Include signal.h,
	sigsetops.h, and stdbool.h.
	(__libc_signal_block_all): Actually block all signals.
	(__libc_signal_block_app): Likewise.
	(__libc_signal_restore_set): Actually restore the signal mask.