parse-options: clearer reporting of API misuse

parse-options: clearer reporting of API misuse

The PARSE_OPT_LASTARG_DEFAULT flag is meant for options like
--contains that (1) traditionally had a mandatory argument and
(2) have some better behavior to use when appearing in the final
position.  It makes no sense to combine this with OPTARG, so ever
since v1.6.4-rc0~71 (parse-options: add parse_options_check to
validate option specs, 2009-07-09) this mistake is flagged with

	error: `--option` uses incompatible flags LASTARG_DEFAULT and OPTARG

and an exit status representing an error in commandline usage.

Unfortunately that which might confuse scripters calling such an
erroneous program into thinking the _script_ contains an error.
Clarify that it is an internal error by dying with a message beginning
"error: BUG: ..." and status 128.

While at it, clean up parse_options_check to prepare for more checks.

Long term, it would be nicer to make such checks happen at compile
time.

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