sniffer: add a mimetype sniffer helper executable

The sniffer is a simple helper process, activated as a DBus service,
that tries to crawl as many files as possible in the provided target
directory (i.e. the new mount's root), for a maximum amount of time -
which is set here to 1.5 seconds (i.e. it will crawl either all the
files in the directory tree, or as many as it can before the specified
timeout expires).

Crawled files are ordered by their content type, and a generic estimation
of the type of files composing the directory is returned to the caller,
using generic 'x-content/*' mimetypes.

The process will then set an autoquit timeout on itself, which can be
disabled by setting the env variable HOTPLUG_SNIFFER_PERSIST for
debugging purposes. The HOTPLUG_SNIFFER_DEBUG env variable can also be
set to enable debugging output.

https://bugzilla.gnome.org/show_bug.cgi?id=653520