base-modem: add setup to allow exclusive operations

There are certain operations that cannot be run in parallel, e.g. one
should not disable and bring the modem to low power mode while there
are also incoming requests to enable the modem. This type of errors
will typically happen when multiple DBus clients are trying to use the
same modem at the same time for different purposes.

We do not have any way to have DBus clients take exclusive ownership
of a given modem object, so the best we can do for now is trying to
synchronize operations that cannot be run together, preserving the
order in which they arrived, and running them one by one.

0 files changed, 0 insertions, 0 deletions