MFC r330349 and r330362:

Allow pause_sbt() to catch signals during sleep by passing C_CATCH flag.
Define pause_sig() function macro helper similarly to other kernel functions
which catch signals. Update outdated function description.

Document pause_sig(9) and update prototypes for existing pause(9) and
pause_sbt(9) functions.

Discussed with: kib@
Suggested by: cem@
Sponsored by: Mellanox Technologies