Clean up old command line parsing

- Move pargs.c to C++ compilation to enable some of the below changes.
- Move *scan() routines from statutil.h/.cpp into pargs.cpp and make
  them static, as this is the only place where they were used.
- Remove vscan() declaration in statutil.h, as the function was not
  defined nor used anywhere.
- Move nenum() declaration from statutil.h to readinp.h, and the
  implementation from statutil.cpp to pargs.cpp.
- Remove unused pdesc() in statutil.cpp.

Clarifies the code somewhat, and removes unnecessary declarations from
headers.

Change-Id: I0b70f3622c8bea41bf88b677cbe202143a3e60c9