This replaces the output parameter (which is bad API design).  As a
side effect, it fixes the bad [[gnu::pure]] attribute added by commit
a636d212 which caused optimizing compilers to miscompile calls to
that function.  "Pure" functions can be assumed to have no output
arguments, so the compiler can assume the function doesn't modify
them.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1282

Forgot that in commit 9a766f4c

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1115

Some users want certain clients to fully control playback, but do not
want them to be able to trigger database update.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1124

make_pair is an old C++98 function that can be replaced by modern
shorter constructs.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The ones in std have overloads for const char/char.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Eliminates a temporary copy.

Closes #296

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

Both callers pass non-nullptr.

As a side effect, this fixes a memory leak.