blockd: make most calls to 'block' asynchronous

blockd: make most calls to 'block' asynchronous

Don't wait for calls to 'block' to complete unless it's for an
autofs event (which cannot be handled async).
Use uloop for 'mount.ready' notificaion when startup has completed
to avoid blocking in waitpid() while the 'block' process is calling
back via ubus.
This greatly reduces the amount of time blockd needs on boot.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>