Introduce per-sockbuf `sb_state' to use it with SS_CANTSENDMORE.

Introduce per-sockbuf `sb_state' to use it with SS_CANTSENDMORE.

This time, socket's buffer lock requires solock() to be held. As a part of
socket buffers standalone locking work, move socket state bits which
represent its buffers state to per buffer state.

Opposing the previous reverted diff, the SS_CANTSENDMORE definition left
as is, but it used only with `sb_state'. `sb_state' ored with original
`so_state' when socket's data exported to the userland, so the ABI kept as
it was.

Inputs from deraadt@.

ok bluhm@