syslog-ng: detect disabled IPv6 on loopback and fallback to IPv4

syslog-ng: detect disabled IPv6 on loopback and fallback to IPv4

Binding in default to IPv6 is preferable but it can be disabled in
kernel and that prevents syslog-ng to start. This setup should not be
that common but syslog is very important service and should survive
that.

This introduces new plugin defining source generator
`network_localhost`. This is used instead of original network source.

Signed-off-by: Karel Kočí <cynerd@email.cz>