uvol: update to 1.2, expose volume operations over ubus

uvol: update to 1.2, expose volume operations over ubus

Move the storage backend probe, the ctx construction, the locking
helpers and the volume-name check out of the CLI script into
/usr/lib/uvol/common.uc, so a second entry point can reuse them
without duplicating the logic.

Publish ubus object 'uvol' with the volume operations and a readiness
query. Consumers such as uxc are pure ubus frontends and must not exec
the command line tool. Ship an rpcd exec plugin, installed by the uvol
package itself, taking the same locks as the CLI so both entry points
stay serialised. As stdout is the plugin's reply channel, point file
descriptor 1 at stderr for the duration of a call.

Send a 'uvol.ready' ubus event at the end of 'uvol boot', carrying the
active backend name and whether the .meta volume is ready. Consumers
such as uxc can wait for this event instead of polling volume state.
Sending is best-effort: boot keeps its exit code even when ubusd is
not reachable.

Signed-off-by: Daniel Golle <redacted>