iptables: Add split out iptables commands

iptables: Add split out iptables commands

Instead of having a pure string based API, we add two new
main functions, __connman_iptables_append() and
__connman_iptables_remove(). The missing commands will be added later.

To simplify the whole code, the __connman_iptables_command() code
is refactored into smaller pieces: parse_rule_spec() calls a few
functions such as clear_tables_flags() and parse_xt_modules()
which should make the reading of the main parser loop simpler.

Also added a few comments on the parser which is really tricky.