This adds a sysctl which allows you to disable the TCP hostcache. This is handy
during testing of network related changes where cached entries may pollute your
results, or during known congestion events where you don't want to unfairly
penalize hosts.

Prior to r232346 this would have meant you would break any connection with a sub
1500 MTU, as the hostcache was authoritative. All entries as they stand today
should simply be used to pre populate values for efficiency.

Submitted by: Jason Wolfe (j at nitrology dot com)
Reviewed by: rwatson, sbruno, rrs , bz (earlier version)
MFC after: 2 weeks
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D6198