luci-app-firewall: replace 'Masquerading' with 'IPv4 Masquerading'

luci-app-firewall: replace 'Masquerading' with 'IPv4 Masquerading'

Although the description in the UI explains that this setting affects
IPv4 only, a more clear label can help some users (hint: me) know that
there the existence of this option does not exclude the existence of
the separate 'IPv6 Masquerading' option.

Signed-off-by: Panagiotis "Ivory" Vasilopoulos <[email protected]>