mtpvolume: fix a memory leak

Make the buffer really static, as the comment above documents.

https://bugzilla.gnome.org/show_bug.cgi?id=723251
mtpvolume: improve volume name

Replace _ and - with spaces, and apply that pass on
the product string too.

https://bugzilla.gnome.org/show_bug.cgi?id=723251
mtpvolume: implement symbolic icons

The places sidebar in nautilus and the places menu in the shell
use symbolic icons exclusively, so make sure one is provided, to
avoid falling back to a generic "network mount" icon.

https://bugzilla.gnome.org/show_bug.cgi?id=723251
various: reduce the verboseness of debug messages

With a recent enough dbus-daemon (or with kdbus), the standard
error of dbus activated services is sent to the journal, so
we should not pollute it with debug messages by default.

https://bugzilla.gnome.org/show_bug.cgi?id=723251