Don't use a leading - in getopt_long() option strings.

The behavior of a leading - is platform-dependent.  It also means that
non-option arguments are treated in a fashion that we're not handling,
so capture filters given as non-option arguments at the end of the
command line don't work.  (The Linux getopt() man page says that a
leading - "is used by programs that were written to expect options and
other argv-elements in any order and that care about the ordering of the
two."  We are not such a program.)

Change-Id: I5610cf90a8218d48f7516abacc367e0affa3b549
Based-On-A-Change-From: Peter Hatina <phatina@redhat.com>
Reviewed-on: https://code.wireshark.org/review/6071
Reviewed-by: Guy Harris <guy@alum.mit.edu>