introduce new icmp-type parameter

introduce new icmp-type parameter

Currently icmp types are handled via 'dport'. This is not documented
anywhere except for a single line of comment in the code. To untangle
the icmp-type handling from the dport handling a new 'icmp-type'
parameter is introduced.

The valid 'icmp-type' values are limited to the names
(icmp[v6]_type_names hash in the code, same as ip[6]tables provides).
Type[/Code] values are not supported.

Support for ipv6-icmp is added to icmp-type parameter handling. This makes it
possible to specify icmpv6 types via the GUI.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>