network: warn when any positive boolean string is specified for IPMasquerade=

Previously, any positive boolean string for IPMasquerade= enables only IPv4
masquerade. The commit 48ed2766 adds
IPv6 masquerade support. However, only "yes" is handled as "ipv4", and other
positive boolean strings are handled as "both".

This makes all positive boolean strings considered as "ipv4", warn that they
are deprecated, and suggest to use "ipv4" or "both".

Follow-up for 48ed2766.