sd-bus: introduce API for re-enqueuing incoming messages

sd-bus: introduce API for re-enqueuing incoming messages

When authorizing via PolicyKit we want to process incoming method calls
twice: once to process and figure out that we need PK authentication,
and a second time after we acquired PK authentication to actually execute
the operation. With this new call sd_bus_enqueue_for_read() we have a
way to put an incoming message back into the read queue for this
purpose.

This might have other uses too, for example debugging.

(cherry picked from commit 1068447e6954dc6ce52f099ed174c442cb89ed54)

zjs: patch modified to not make the function public
(cherry picked from commit 83bfc0d8dd026814d23e3fdfa46806394f775526)
(cherry picked from commit 2e504c92d195d407cec3ba9ed156b195c31a5f3f)
(cherry picked from commit 351627d4bfa39dd05f28d889967383af2372de6d)

Gbp-Pq: Name sd-bus-introduce-API-for-re-enqueuing-incoming-messages.patch