Move some more stuff into wsutil.

Move the routines to parse numerical command-line arguments there.

Make cmdarg_err() and cmdarg_err_cont() routines in wsutil that just
call routines specified by a call to cmdarg_err_init(), and have
programs supply the appropriate routines to it.

Change-Id: Ic24fc758c0e647f4ff49eb91673529bcb9587b01
Reviewed-on: https://code.wireshark.org/review/2704
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Guy Harris <guy@alum.mit.edu>