Add new Tramp method "media"

Add new Tramp method "media"

* doc/misc/tramp.texi (Quick Start Guide, GVFS-based methods):
Add media devices.

* etc/NEWS: Mention new Tramp method "media".

* lisp/net/tramp-gvfs.el (tramp-gvfs-methods): Add "media" method.
(tramp-goa-methods): Add tramp-autoload cookie.
(tramp-media-methods): New defvar.
(tramp-gvfs-service-volumemonitor): New defsubst.
(top): Remove media methods if not supported.  Add defaults for
`tramp-default-host-alist'.
(tramp-goa-account): Rename from `tramp-goa-name'.  Adapt all callees.
(tramp-gvfs-service-afc-volumemonitor)
(tramp-gvfs-service-goa-volumemonitor)
(tramp-gvfs-service-gphoto2-volumemonitor)
(tramp-gvfs-service-mtp-volumemonitor)
(tramp-gvfs-path-remotevolumemonitor)
(tramp-gvfs-interface-remotevolumemonitor): New defconsts.
(tramp-media-device): New defstruct.
(tramp-gvfs-activation-uri): New defun.
(tramp-gvfs-url-file-name): Use it.
(tramp-gvfs-handler-mounted-unmounted)
(tramp-gvfs-connection-mounted-p, tramp-gvfs-mount-spec):
Handle "media" method.
(tramp-get-goa-account): Rename from `tramp-make-goa-name'.  Adapt
all callees.
(tramp-get-goa-accounts): Adapt docstring.  Cache with nil key.
(tramp-parse-goa-accounts, tramp-get-media-device)
(tramp-get-media-devices)
(tramp-parse-media-names): New defuns.
(top): Rework completion function registration.

* lisp/net/tramp.el (tramp-dns-sd-service-regexp): New defconst.
(tramp-set-completion-function): Use it.