sh: Add test for the non-standard jobid builtin.
sh: Recognize "--" as end of options in bg/fg/jobid builtins.
pselect(2): Add xref to sigsuspend(2).
Restore the previous sendfile(2) behaviour on the block devices.
Provide valid .fo_sendfile method for several missed struct fileops.

Reviewed by:	glebius
Sponsored by:	The FreeBSD Foundation