fs: Draft module to mount filesystem representing navymail store

fs: Draft module to mount filesystem representing navymail store

Now one can mount navymail store, and access gboxes with usual mail clients,
such as mutt. Only read-only access is implemented.

The filesystem is implemented as a high-level FUSE module which exports gbox'es
in traditional mbox format, i.e. files content is the same as output of
`navymail export --original-order`.

In order to measure fuse overhead, I've timed plain `navymail export` and cat
of the same gbox from filesystem for sup-talk archives:

	navymail export --original-order sup-talk >/dev/null	0.69s
	cat <mountpoint>/mail/sup-talk >/dev/null		0.75s

So overhead the overhead is ~ 10%.

NOTE while implementing first draft I did not pay attention to simultaneous
     access and locking, only tried to get the thing up and running.

Signed-off-by: Kirill Smelkov <kirr@navytux.spb.ru>