gdbus: Remove proxies when client disconnects

Remove proxies and generate proxy_removed callbacks even when there are
no corresponding InterfaceRemoved signals.

This patch fixes having zombie gdbus proxy object when a server
disconnects without sending InterfaceRemoved signals. These objects
may interact with new server instances, for example, making
InterfaceAdded signals of new objects with the same name be filtered
out as duplicated, or staying allocated, but unused, if the new server
doesn't reuse the object paths. Note that as a side-effect, the
lifetime of a gdbus proxy becomes stricter: it lives at most for the
duration of a single connection to a single instance of a server
process.
mgmt: Add support for new MGMT_STATUS_RFKILLED
core: Use a dedicated D-Bus error for rfkill power on failure
shared: Add API to get access to the hciemu bthost instance
tools/l2cap-tester: Take advantage of the new hciemu_client_get_host API
shared: Remove unused hciemu functions
emulator: Return used L2CAP ident value from bthost_l2cap_cmd()
emulator: Make bthost L2CAP sender strictly only for requests
emulator: Add callback mechanism for L2CAP requests
emulator: Add handler for L2CAP command reject PDU
tools/l2cap-tester: Add invalid L2CAP PDU test case
obexd: Add process callback to session's pending_request

This adds a process callback and a data destroy callback to the session's
pending_request structure.

This is needed as preparation of queuing all OBEX session commands.
obexd: Add session_queue

Add helper function session_queue that appends a pending_request to the
session's command queue.
obexd: Use process callback to queue transfers

This generalizes the existing queuing of transfers by using the
process callback of pending_request.
obexd: Queue setpath command
obexd: Add file_data struct to session

This adds a common file_data struct and related free and complete callbacks
to the session. This will be used for queuing the file related commands
mkdir, copy, move and delete.
obexd: Queue mkdir command
obexd: Queue copy command
obexd: Queue move command
obexd: Queue delete command