luci-app-firewall: Add 'any' choice for SNAT 'family' option

If one sets a SNAT rule via the GUI as 'automatic', the
'family' remains empty. In fw4.uc code, this is interpreted as:

/* default to IPv4 only for backwards compatibility,
 unless an explicit family any was configured */

'any' is handled by fw4 as IPv4+6.

Also prevent 'any' from triggering a validation error (non-SNAT targets
hide 'snat_ip' which remains empty, and triggered an error).

Signed-off-by: Paul Donald <newtwen+github@gmail.com>