main: Fix for misleading error with negative chain priority

main: Fix for misleading error with negative chain priority

getopt_long() would try to parse the negative priority as an option and
return -1 as it is not known:

| # nft add chain x y { type filter hook input priority -30\; }
| nft: invalid option -- '3'

Fix this by prefixing optstring with a plus character. This instructs
getopt_long() to not collate arguments but just stop after the first
non-option, leaving the rest for manual handling. In fact, this is just
what nft desires: mixing options with nft syntax leads to confusive
command lines anyway.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>