threads: call `vlc_cond_t` signal/broadcast while locked
MR following the discussion here: !4883 (comment 425357)
One less vlc_mutex_t
lock
/unlock
for each vlc_cond_t
wait
/signal
/broadcast
call.
Most VLC codebase was already calling vlc_cond_t
signal
/broadcast
with a lock held, except in the 10 first commits of this MR.
Edited by Thomas Guillem