liblog: logcat: add epoch and monotonic format modifiers

- '-v epoch' prints seconds since Jan 1 1970
- '-v monotonic' print cpu seconds since start of device
- '-T sssss.mmm...' as alternate tail time format

NB: monotonic is a best estimate and may be out by a few ms
    given the synchronization source clue accuracy.

Bug: 23668800
Change-Id: Ieb924b6d3817669c7e53beb9c970fb626eaad460