xshared: Fix parsing of option arguments in same word

xshared: Fix parsing of option arguments in same word

When merging commandline parsers, a decision between 'argv[optind - 1]'
and 'optarg' had to be made in some spots. While the implementation of
check_inverse() required the former, use of the latter allows for the
common syntax of '--opt=arg' or even '-oarg' as 'optarg' will point at
the suffix while 'argv[optind - 1]' will just point at the following
option.

Fix the mess by making check_inverse() update optarg pointer if needed
so calling code may refer to and always correct 'optarg'.

Fixes: 0af80a91b0a98 ("nft: Merge xtables-arp-standalone.c into xtables-standalone.c")
Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1677
Signed-off-by: Phil Sutter <phil@nwl.cc>