logcat: -f flag to continue

On startup, check the current logging content, then grab logs from
that time forward rather than restarting from the beginning. Add
support for reading tail time down to the nano-second.

Bonus, permits us to create a logcatd logpersist daemon

Bug: 19608716
Change-Id: Iaab58de4c02b7f97c69de12cf954fedc2163059e