Simplify unix socket connection peer locking.

Simplify unix socket connection peer locking.

unp_pcb_owned_lock2() has some sharp edges and forces callers to deal
with a bunch of cases.  Simplify it:

- Rename to unp_pcb_lock_peer().
- Return the connected peer instead of forcing callers to load it
  beforehand.
- Handle self-connected sockets.
- In unp_connectat(), just lock the accept socket directly.  It should
  not be possible for the nascent socket to participate in any other
  lock orders.
- Get rid of connect_internal().  It does not provide any useful
  checking anymore.
- Block in unp_connectat() when a different thread is concurrently
  attempting to lock both sides of a connection.  This provides simpler
  semantics for callers of unp_pcb_lock_peer().
- Make unp_connectat() return EISCONN if the socket is already
  connected.  This fixes a race[1] when multiple threads attempt to
  connect() to different addresses using the same datagram socket.
  Upper layers will disconnect a connected datagram socket before
  calling the protocol connect's method, but there is no synchronization
  between this and protocol-layer code.

Reported by:	syzkaller [1]
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D26299