Move parse_common_args() into commandline/pargs.*

- Remove statutil.* as empty after the move.
- Remove unnecessary inclusions of statutil.h.
- Replace remaining inclusions with either pargs.h, or one of the
  headers previously included by statutil.h.
- Make get_pargs() internal to pargs.cpp, as it is no longer called from
  outside the file.

Change-Id: Ie171eff303596ae3eb254d505d8c8b84dff1f84f