kernel/signal.c: fix typos and coding style

General coding style and comment fixes; no code changes:

 - Use multi-line-comment coding style.
 - Put some function signatures completely on one line.
 - Hyphenate some words.
 - Spell Posix as POSIX.
 - Correct typos & spellos in some comments.
 - Drop trailing whitespace.
 - End sentences with periods.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>