Fix some theoretical races in signal handling

Fix some theoretical races in signal handling

Problem reported by Johannes Przybilla (Bug#32375).
* NEWS: Mention this.
* bootstrap.conf (gnulib_modules): Add sigaction.
* gzip.c (SA_NOCLDSTOP, sigprocmask, sigset_t)
(siginterrupt) [!SA_NOCLDSTOP]: Remove; Gnulib not supplies these.
(remove_ofname): New var.
(volatile_strcpy): New function.
(create_outfile): Use it.
(install_signal_handlers, abort_gzip_signal): Assume sigaction.
(remove_output_file): New arg SIGNALS_ALREADY_BLOCKED.
All uses changed.  Avoid unnecessary (and racy) call
to sigprocmask if this new arg is set.
(abort_gzip_signal): Assume C89 or better for signal handler type.
* gzip.h (RETSIGTYPE): Remove.
* lib/.gitignore, m4/.gitignore:
Add files brought in by Gnulib sigaction module.  Sort.