Introduce SBL_WAIT and SBL_NOINTR sbwait() flags.

Introduce SBL_WAIT and SBL_NOINTR sbwait() flags.

This refactoring is another step to make standalone socket buffers
locking. sblock() uses M_WAITOK and M_NOWAIT flags passed as the third
argument together with the SB_NOINTR flag on the `sb_flags' to control
sleep behaviour. To perform uninterruptible acquisition, SB_NOINTR flag
should be set before sblock() call. `sb_flags' modification requires to
hold solock() around sblock()/sbunlock() that makes standalone call
impossible.

Also `sb_flags' modifications outside sblock()/sbunlock() makes
uninterruptible acquisition code huge enough. This time only sorflush()
does this (and forgets to restore SB_NOINTR flag, so shutdown(SHUT_RDWR)
call permanently modifies socket locking behaviour) and this looks not
the big problem. But with the standalone socket buffer locking it will
be many such places, so this huge construction is unwanted.

Introduce new SBL_NOINTR flag passed as third sblock() argument. The
sblock() acquisition will be uninterruptible when existing SB_NOINTR
flag is set on `sb_flags' or SBL_NOINTR was passed.

The M_WAITOK and M_NOWAIT flags belongs to malloc(9). It has no M_NOINTR
flag and there is no reason to introduce it. So for consistency reasons
introduce new SBL_WAIT and use it together with SBL_NOINTR instead of
M_WAITOK and M_NOINTR respectively.

ok bluhm