ubusd: attempt to create socket folder

ubusd: attempt to create socket folder

When ubus is running as root, /var/run/ubus most likely hasn't been
created as well (as that's the homedir of user ubus, and if a user
ubus was found, then ubus would run being that user).
Blindly attempt to create the directory (which won't do any harm if
it does exist and/or ubus is not running as root) to still be able to
start ubus in case of user ubus not existing.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>