ptp clock: Wait for ANNOUNCE before selecting a master

ptp clock: Wait for ANNOUNCE before selecting a master

Previously, with opportunistic sync we'd track a master
clock as soon as we see a SYNC message, and hence sync up
faster, but then we'd announce we're synched before seeing
the ANNOUNCE, leaving the clock details like grandmaster-clock
empty.

A better way is to start tracking the clock opportunistically,
but not announce we're synched until we've also seen the ANNOUNCE.