main: enforce options before commands

main: enforce options before commands

This patch turns on POSIXLY_CORRECT on the getopt parser to enforce
options before commands. Users get a hint in such a case:

 # nft list ruleset -a
 Error: syntax error, options must be specified before commands
 nft list ruleset -a
    ^             ~~

This patch recovers 9fc71bc6b602 ("main: Fix for misleading error with
negative chain priority").

Tests have been updated.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>