compat/linux: map TCP_USER_TIMEOUT sockopt into TCP_MAXUNACKTIME

After reading both manual pages, our TCP_MAXUNACKTIME is fairly
similar to the TCP_USER_TIMEOUT, the only considerable difference
is ours is in seconds and linux's in milliseconds.

Round up linux's in setsockopt(2) to a next whole second and
clamp ours getter to UINT_MAX ms.

Reviewed by: tuexen, glebius
Differential Revision: https://reviews.freebsd.org/D56168
MFC after: 2 weeks
Sponsored by: Sippy Software, Inc.