- csplit sometimes failed to remove files when interrupted.

 - csplit sometimes failed to remove files when interrupted.
 - csplit didn't clean up if two signals arrived nearly simultaneously.

(sigprocmask, sigset_t) [!defined SA_NOCLDSTOP]: Define.
(filename_space, prefix, suffix, digits, files_created, remove_files): Now volatile.
(caught_signals): New var.
(cleanup): Block signals while deleting all files.
(cleanup_fatal, handle_line_error, regexp_error):
Mark with ATTRIBUTE_NORETURN.
(create_output_file, close_output_file, interrupt_handler):
Block signals while changing the number of output files,
to fix some race conditions.
(delete_all_files): Do nothing if remove_files is zero.
Clear files_created.
(main): Don't mess with signals until after argument processing is done.

(main): Rewrite signal-catching code to make it
similar to other coreutils programs.  When processing signals,
block all signals that we catch, but do not block signals that we
don't catch.  Avoid problems with unsigned int warnings.

(interrupt_handler): Use void, not (obsolete) RETSIGTYPE.

(interrupt_handler) [defined SA_NOCLDSTOP]:
Use simpler "signal (sig, SIG_DFL)" rather than sigaction equivalent.