Replace struct tcpopt with a proxy toeopt struct in the TOE driver interface to
the TCP syncache. This returns struct tcpopt to being private within the TCP
implementation, thus allowing it to be modified without ABI concerns.

The patch breaks the ABI. Bump __FreeBSD_version to 800103 accordingly. The cxgb
driver is the only TOE consumer affected by this change, and needs to be
recompiled along with the kernel.

Suggested by: rwatson
Reviewed by: rwatson, kmacy
Approved by: re (kensmith), kensmith (mentor temporarily unavailable)