Refactor sleepq_catch_signals().

Refactor sleepq_catch_signals().

- Extract suspension check into sig_ast_checksusp() helper.
- Extract signal check and calculation of the interruption errno into
  sig_ast_needsigchk() helper.
The helpers are moved to kern_sig.c which is the proper place for
signal-related code.

Improve control flow in sleepq_catch_signals(), to handle ret == 0
(can sleep) and ret != 0 (interrupted) only once, by separating
checking code into sleepq_check_ast_sq_locked(), which return value is
interpreted at single location.

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D26628