update-index: migrate to parse-options API

--refresh and --really-refresh accept flags (like -q) and modify
an error indicator.  It might make sense to make the error
indicator global, but just pass the flags and a pointer to the error
indicator in a struct instead.

--cacheinfo wants 3 arguments.  Use the OPTION_LOWLEVEL_CALLBACK
extension to grab them and PARSE_OPT_NOARG to disallow the "sticked"
--cacheinfo=foo form.  (The resulting message

	$ git update-index --cacheinfo=foo
	error: option `cacheinfo' takes no value

is unfortunately incorrect.)

--assume-unchanged and --no-assume-unchanged probably should use the
OPT_UYN feature; but use a callback for now so the existing MARK_FLAG
and UNMARK_FLAG values can be used.

--stdin and --index-info are still constrained to be the last argument
(implemented using the OPTION_LOWLEVEL_CALLBACK extension).

--unresolve and --again consume all arguments that come after them
(also using OPTION_LOWLEVEL_CALLBACK).

The order of options matters.  Each path on the command line is
affected only by the options that come before it.  A custom
argument-parsing loop with parse_options_step() brings that about.

In exchange for all the fuss, we get the usual perks: support for
un-sticked options, better usage error messages, more useful -h
output, and argument parsing code that should be easier to tweak
in the future.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>