samba36: append config from /var/run/config/ for runtime shares

samba36: append config from /var/run/config/ for runtime shares

This will allow automation/hotplug.d scripts to store runtime shares in
the /var/run/config/samba. It's useful e.g. for USB drives that user
wants to be automatically shared.

Using /var/run/config/ provides:
1) Automated cleaning on reboots
   It's important for consistency (to avoid sharing non-existing drives)
2) Safety for user non-commited changes
   Automated scripts should never call "uci [foo] commit" as that could
   flush incomplete config.

Another minor gain is avoiding flash wearing for runtime setup.

Cc: Rosy Song <rosysong@rosinson.com>
Cc: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>