Simplify by using Gnulib sigdescr_np module

Simplify by using Gnulib sigdescr_np module

Inspired by a straightforward patch by Bruno Haible.
* admin/merge-gnulib (GNULIB_MODULES): Add sigdescr_np.
* configure.ac: Do not check for sys_siglist or __sys_siglist.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* lib/sigdescr_np.c, m4/sigdescr_np.m4: New files, copied from Gnulib.
* src/sysdep.c (sys_siglist, sys_siglist_entries): Remove.
(init_signals): Do not initialize sys_siglist.
(safe_strsignal): Use sigdescr_np instead of sys_siglist.