Rename nptl-signals.h to internal-signals.h

Rename nptl-signals.h to internal-signals.h

This patch renames the nptl-signals.h header to internal-signals.h.
On Linux the definitions and functions are not only NPTL related, but
used for other POSIX definitions as well (for instance SIGTIMER for
posix times, SIGSETXID for id functions, and signal block/restore
helpers) and since generic functions will be places and used in generic
implementation it makes more sense to decouple it from NPTL.

Checked on x86_64-linux-gnu.

	* sysdeps/nptl/nptl-signals.h: Move to ...
	* sysdeps/generic/internal-signals.h: ... here.  Adjust internal
	comments.
	* sysdeps/unix/sysv/linux/internal-signals.h: Add include guards.
	(__nptl_is_internal_signal): Rename to __is_internal_signal.
	(__nptl_clear_internal_signals): Rename to __clear_internal_signals.
	* sysdeps/unix/sysv/linux/raise.c: Adjust nptl-signal.h to
	include-signals.h rename.
	* nptl/pthreadP.h: Likewise.
	* sysdeps/unix/sysv/linux/spawni.c (__spawni_child): Call
	__is_internal_signal instead of __nptl_is_internal_signal.