core: add new setting Writable= to ListenSpecial= socket units

Writable= is a new boolean setting. If ture, then ListenSpecial= will
open the specified path in O_RDWR mode, rather than just O_RDONLY.

This is useful for implementing services like rfkill, where /dev/rfkill
is more useful when opened in write mode, if we want to not only save
but also restore its state.