sockets: make pr_shutdown fully protocol specific method

[glebius]

Summary

Disassemble a one-for-all soshutdown() into protocol specific methods.
This creates a small amount of copy & paste, but makes code a lot more
self documented, as protocol specific method would execute only the code
that is relevant to that protocol and nothing else. This also fixes a
couple recent regressions and reduces risk of future regressions. The
extended KPI for the new pr_shutdown removes need for the extra pr_flush
which was added for the sake of SCTP which could not perform its shutdown
properly with the old one. Particularly for SCTP this change streamlines
a lot of code.

Some notes on why certain parts of code were copied or were not to certain
protocols:

• The (SS_ISCONNECTED | SS_ISCONNECTING | SS_ISDISCONNECTING) check is needed only for those protocols that may be connected or disconnected.
• The above reduces into only SS_ISCONNECTED for those protocols that always connect instantly.
• The ENOTCONN and continue processing hack is left only for datagram protocols.
• The SOLISTENING(so) block is copied to those protocols that listen(2).
• sorflush() on SHUT_RD is copied almost to every protocol, but that will be refactored later.
• wakeup(&so->so_timeo) is copied to protocols that can make a non-instant connect(2), can SO_LINGER or can accept(2).

There are three protocols (netgraph(4), Bluetooth, SDP) that did not have
pr_shutdown, but old soshutdown() would still perform sorflush() on
SHUT_RD for them and also wakeup(9). Those protocols partially supported
shutdown(2) returning EOPNOTSUP for SHUT_WR/SHUT_RDWR, now they fully lost
shutdown(2) support. I'm pretty sure netgraph(4) and Bluetooth are okay
about that and SDP is almost abandoned anyway.

[tuexen]

I like the changes. Once this hits the tree, I need to upstream it. This might result in undoing some whitespace changes, where you insert line breaks to enforce a 80 character line limit and I might use KASSERT instead of MPASS, since I think I don't use MPASS in the userland stack. But I can deal with this.
I really like making shutdown() protocol specific. This allows discussing TCP or SCTP specific behavior. In some case, I think, a different behavior in some error cases would make sense. But I always did not start discussing it, since this would have touched several protocols.