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>

Closes https://github.com/MusicPlayerDaemon/MPD/issues/465 and
https://github.com/MusicPlayerDaemon/MPD/pull/466

Modeled after std::filesystem::operator/() from C++17.

Turns out that using CP_ACP is a lousy idea, because only very few
Unicode characters can be represented by it.  Instead, switch to UTF-8
(which every sane person on other operating system already uses).

Closes #102

Prepare to add more modes.

As a side effect, "playlistclear" no longer creates a new playlist if
the given one doesn't yet exist.