amlgrep: drop mawk support, use RT for ORS

* Now only supports gawk
* Output RT (Record Terminator; actual match strings of RT), instead of
  fixed default ORS string, unless specified by --ors
* New --timestamp option to handle certain types of multi-line logs,
  in which entries begins with something like '\n2014-12-31 12:34:56'
  (This option really is just a wrapper for --rs option)