Change a few members of tcpcb that store cached copies of ticks to be ints

Change a few members of tcpcb that store cached copies of ticks to be ints
instead of unsigned longs.  This fixes a few overflow edge cases on 64-bit
platforms.  Specifically, if an idle connection receives a packet shortly
before 2^31 clock ticks of uptime (about 25 days with hz=1000) and the keep
alive timer fires after 2^31 clock ticks, the keep alive timer will think
that the connection has been idle for a very long time and will immediately
drop the connection instead of sending a keep alive probe.

Reviewed by:	silby, gnn, lstewart
MFC after:	1 week