Fixes gcc5 warning.

Return false on error, not nullptr.

Fixes -Winconsistent-missing-override (clang 3.6).

New in clang 3.6.

The definition of the playlist_plugin struct member of the embcue
plugin was incorrect.

Works around assertion failure due to something that appears to be a
(minor) FFmpeg bug.

Version 2.5 fixed an API oddity, however it broke API compatibility,
at least with C++.  Disable the workaround when a libavformat version
is detected that is recent enough.

The "::" to explicitly refer to the global namespace appeared like a
good idea in C++, but it breaks with C libraries that implement
standard functions using macros (e.g. musl).

Set the Error when zzip_seek()==-1 and not on success.  Fixes a crash
after seeking.

A negative uid value means it's not a "local socket" (PF_LOCAL).
uid==0 means user "root" connected.

Must return -1 on error, not 0.  0 is root.

Use the new uri_get_suffix() overload that removes the query string.

Many years ago, FAAD had a serious ABI bug: the NeAACDecInit()
prototype in its header declared the "samplerate" parameter to be
"unsigned long *", but internally, the function assumed it was
"uint32_t *" instead.  On 32 bit machines, that was no difference, but
on 64 bit, this left one portion of the return value uninitialized;
and worse, on big-endian, the wrong word was filled.  This bug had to
be worked around in MPD (commit 9c4e97a6).

A few months later, the bug was fixed in the FAAD CVS in commit 1.117
on file libfaad/decoder.c; the commit message was:

 "Use public headers internally to prevent duplicate declarations"

The commit message was too brief at best; the problem was not
duplicate declarations, but a prototype mismatch.  No mention of the
bug fix in the ChangeLog.

The MPD project never learned about this bug fix, and so MPD would
always pass a "uin32_t *" dressed up as a "unsigned long *".  Nearly 6
years later, it's about time to fix this second ABI problem.  Let's
kill the workaround!

A mutex acts as a memory barrier, and thus "volatile" is not
necessary.