Some wording improvements

Signed-off-by: Rafaël Carré <funman@videolan.org>

(untested as I have no software capable of sending this)

Qt4 (and a bunch of other GUIs, I suspect) needs to reference the audio
output properly, but they don't. As such, we need to keep the loosy,
slow and broken vlc_object_get() usage.

This reverts commit 7f120f79.

Signed-off-by: Rémi Denis-Courmont <rdenis@simphalempin.com>

Unused video outputs are currently attached to libvlc, so there is no
point in destroying them from the playlist (twice), then from libvlc.
This should also fix #1593 (multiple vout free).

(assuming rt-offset is zero)

POSIX warrants at least 32 priorities from
sched_get_priority_min(SCHED_RR) to sched_get_priority_max(SCHED_RR).
We were previously relying on 41 priorities, from 0 to 40.

Contrary to process priority increment ("niceness"), thread priority
increment increase in the priority of the thread. So multiplying
negative priorities with -1 was totally wrong here.

On Linux, there is only one non-realtime (SCHED_OTHER) priority, 0, so
the code was failing safe silently. Don't know on other systems.