This reverts commit 4475b8ca.  Further
testing revealed that the threaded resolver still uses a timeout of
0ms.  This revert however lowers the bound to a minimum of 1ms instead
of 10ms.

curl_multi_remove_handle() calls our socket function, and there's no
need to call curl_multi_socket_action().

This was a problem 9 years ago, and apparently, it has been fixed long
ago.

For simplicity, this commit removes a workaround for an old CURL bug.

Prepare the transition to std::mutex.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

This way, IsFileNotFound() can detect status 404.

Not needed on Android, and the implementation uses getpwuid_r() which
is unavailable on old Android versions.