[review] allow parallel Enable() or Disable() requests

[Aleksander Morgado]

Hey all,

See this MR: https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_requests/19

It includes some changes to allow running multiple concurrent Enable()
requests. All the subsequent attempts after the first one are queued
until the first one finishes, and they will all share the same
operation result and be completed at the same time. And same thing for
Disable().

What do you think?

-- 
Aleksander
https://aleksander.es