This adds basic systemd support, incl. socket activation.
If systemd build is disabled, there won't be any sign of it neither
during build nor after.
If systemd build is enabled (default), but systemd is not available,
no systemd stuff is installed, so there is no impact whatsoever.
this is mainly useful to have a single location to read the setting
from, when it is used multiple times at build time (which it will in a
following patch) :)
rename the ADD_WATCH/REMOVE_WATCH messages to SUBSCRIBE/UNSUBSCRIBE and change
the message format and libubus API in preparation for adding object notifications
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
