Allows signalling a failed seek, and replaces several redundant
command checks after the ProcessCommand() call.

Decouple this function from player command execution.

Thsi filter doesn't make sense as manually configured plugin, because
nobody ever calls volume_filter_set() on it, making it a no-op.

These are not in filter_plugins[] and have dedicated constructors
anyway.

Supporting GCC 4.9 is becoming cumbersome as we refactor more code to
C++14.

This shouldn't be a problem for anybody, because Debian Stable
(Stretch) has version 6.3, and Ubuntu LTS (Xenial) has version 5.3.

This resolves the circular dependency between IcyInputStream and
CurlInputStream.

Similar to commit 31ab78ae

Fixes deadlock because FileInputStream::Read() unlocks the mutex
(which was not locked) and then locks it, keeping it locked.  This can
result in a deadlock.  This happens because the archive and the file
mutex are different.

Fixes thread sanitizer warnings.

Fixes thread sanitizer warnings.

Use the "==" operator instead of pthread_equal().

This allows us to eliminate two boolean flags which are there to avoid
race conditions, and made the thing so fragile that I got tons of
(correct) thread sanitizer warnings.

Use one single std::lock_guard in Run(), and replace the manual
Lock()/Unlock() calls.

Eliminate more unlock/lock transitions, making the code more robust.

Regression from commit 0772e571