Move SB_SPLICE, SB_WAIT and SB_SEL to `sb_flags', serialized by solock().

Move SB_SPLICE, SB_WAIT and SB_SEL to `sb_flags', serialized by solock().

SB_KNOTE remains the only bit set on `sb_flagsintr' as it is set/unset in
contexts related to kqueue(2) where we'd like to avoid grabbing solock().

While here add some KERNEL_LOCK()/UNLOCK() dances around selwakeup() and
csignal() to mark which remaining functions need to be addressed in the
socket layer.

ok visa@, bluhm@