tcp: Don't prematurely drop receiving-only connections.

tcp: Don't prematurely drop receiving-only connections.

If the connection was persistent and receiving-only, several (12)
sporadic device insufficient buffers would cause the connection be
dropped prematurely:
Upon ENOBUFS in tcp_output() for an ACK, retransmission timer is
started.  No one will stop this retransmission timer for receiving-
only connection, so the retransmission timer promises to expire and
t_rxtshift is promised to be increased.  And t_rxtshift will not be
reset to 0, since no RTT measurement will be done for receiving-only
connection.  If this receiving-only connection lived long enough,
and it suffered 12 sporadic device insufficient buffers, i.e.
t_rxtshift >= 12, this receiving-only connection would be dropped
prematurely by the retransmission timer.

We now assert that for data segments, SYNs or FINs either rexmit or
persist timer was wired upon ENOBUFS.  And don't set rexmit timer
for other cases, i.e. ENOBUFS upon ACKs.

And we no longer penalize send window upon ENOBUFS.

Obtained-from: FreeBSD r300981