multipath: add "count paths" multipathd command

multipath: add "count paths" multipathd command

This adds a new multipathd command, "count paths". which returns information in
the format

Paths: <nr_of_paths>
Busy: <True|False>

where "Paths" is the number of monitored paths, and "Busy" is set when
multipathd is currently handling uevents.  With this, it is possible to quickly
get the number of paths being monitored, as well as an idea if more paths may
be showing up shortly.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>