Merge branch 'jc/parseopt-command-modes'

Many commands use --dashed-option as a operation mode selector
(e.g. "git tag --delete") that the user can use at most one
(e.g. "git tag --delete --verify" is a nonsense) and you cannot
negate (e.g. "git tag --no-delete" is a nonsense).  Make it easier
for users of parse_options() to enforce these restrictions.

* jc/parseopt-command-modes:
  tag: use OPT_CMDMODE
  parse-options: add OPT_CMDMODE()