Add a sysctl, net.inet.icmp.tstamprepl, which can be used to disable replies
to ICMP Timestamp packets.

PR:		193689
Submitted by:	Anthony Cornehl <accornehl@gmail.com>
MFC after:	3 weeks
Sponsored by:	EMC / Isilon Storage Division


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272378 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f