Add a sysctl variable ts_offset_per_conn to change the computation

Add a sysctl variable ts_offset_per_conn to change the computation
of the TCP TS offset from taking the IP addresses and the TCP port
numbers into account to a version just taking only the IP addresses
into account. This works around broken middleboxes or endpoints.
The default is to keep the behaviour, which is also the behaviour
recommended in RFC 7323.

Reported by:		devgs@ukr.net
Reviewed by:		rrs@
MFC after:		2 weeks
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D20980