MFC 245238:

MFC 245238:
Add an option to not drop options from the third retransmitted SYN.  If the
SYNs (or SYN/ACK replies) are dropped due to network congestion, then the
remote end of the connection may act as if options such as window scaling
are enabled but the local end will think they are not.  This can result in
very slow data transfers in the case of window scaling disagreements.

Note that the unlike HEAD the existing behavior is preserved by default,
but it can be disabled by setting the
net.inet.tcp.rexmit_drop_options sysctl to zero.

git-svn-id: svn://svn.freebsd.org/base/stable/8@247499 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f