fw4: add support for `option log` in rule and redirect sections

fw4: add support for `option log` in rule and redirect sections

Sections of type `rule` and type `redirect` may now specify
`option log value` to enable logging matched traffic for the
corresponding rule/redirect.

The value may be either a string, in which case it is used as log prefix
verbatim or a boolean value (`1`, `on`, `true`, `yes`, `0`, `off`, `false`
or `no`).

In case a boolean false value is specified (the default), no logging is
performed. In case a true boolean value is specified, matched traffic is
logged and the rule's name (or uci section id i ncase the name is absent)
is used as log prefix.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>