Cosmetic fixes, no code changes

Cosmetic fixes, no code changes

* defs.h: Add/reformat comments.
* signal.c: Remove wrong comment. Add warning directive
when we detect that NSIG is undefined. Add comment about
NSIG on ARM. Fix typo in comment.
(signame): Reformat code a bit without changes to logic.
Shorten static buffer.
(sys_rt_sigprocmask): Remove stray empty line.
* syscall.c: Add warning directive when we detect that
NSIG is undefined. Add comment about NSIG on ARM.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>