Fixes missing song length on high-latency Opus files.

According to tests with 320 kbit/s opus files with 60ms packets, we
need to skip up to 29 kB.

Turns out the lock-free code using atomics was not thread-safe.  The
given callback could be invoked by GLib before the source_id attribute
was assigned.  This commit changes the DeferredMonitor class to use a
Mutex to block the event loop until source_id is assigned.  This bug
does not exist in the 0.19 branch because it does not use the GLib
main loop anymore.

This corrects a major mistake from commit 724a59aa - there was one
small thing that commit was supposed to do, and it failed.
AV_TIME_BASE is not a seek flag.

Has been superseded by the previous commit.

Don't wait for an optimistic write to fail.  This is an improved
workaround for the infamous Raspberry Pi bug (see commit af991765).
It works much better and comes without the negative side effects.  The
old workaround is now obsolete.

Fixes hanging child process in the "pipe" output plugin.

Link libvorbisfile first, followed to libvorbis and finally libogg.
This order is necessary because libvorbisfile depends on libvorbis.

Fixes another linker failure.  Similar to commit ea406875

Since the encoder plugin uses a libvorbis function (and not only
libvorbisenc functions), we need to link with libvorbis explicitly.

Allow semicolons and single quotes in the stream title.  This is not
part of any specification, but found in real life.

Works around a build failure on Solaris because annoyingly, Solaris
reserves the name "queue".  This rename was pending anyway.

Fixes potential nullptr dereference.

Fixes clang warning.

Look for symbol "mpc_demux_init" which does not exist in SV7.  This
avoids build failures when SV7 was found by configure.ac.