Persist timers TCPTV_PERSMIN and TCPTV_PERSMAX are hardcoded with 5 seconds and
60 seconds, respectively. Turn them into sysctls that can be tuned live. The
default values of 5 seconds and 60 seconds have been retained.

Submitted by: Jason Wolfe (j at nitrology dot com)
Reviewed by: gnn, rrs, hiren, bz
MFC after: 1 week
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D5024