um: daemon: Make default socket configurable

Even if daemon network is deprecated, some configurations may
still use it (e.g. Debian), and not want to default to the
/tmp/uml.ctl socket location. Allow configuring the default
socket location.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Tested-by: Ritesh Raj Sarraf <ritesh@collabora.com>
Signed-off-by: Richard Weinberger <richard@nod.at>