Found with readability-uppercase-literal-suffix
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Merge some duplicate code in a central library.

This can cause request completion in the I/O thread before this
constructor returns, leaving the object in an abstract state, causing
a crash due to pure virtual method call.  We should not start the
request until this object is fully constructed.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/665

This is the last missing piece for https://github.com/MusicPlayerDaemon/MPD/issues/662

Commit 29f78b18 continued.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/662

Without requesting the property, "good" WebDAV servers would not send
it, and so MPD could never recognize a directory, failing the database
update.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/660

Even if the LocalStorage is available, return the "udisks://" URI when
the MapUTF8() parameter is an empty string.  This fixes the mount URI
in the state file.

Closes #487

Fixes the "org.freedesktop.UDisks2.Error.AlreadyMounted" error.

Closes #485

Closes #477

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!

Since nfs_stat_async is deprecated.

When using a database that was not created with a WebDAV music_directory
(i.e., if using a remote database, on which updates happen locally) and
using the Curl storage plugin, MPD would previously send GET requests that
had unescaped spaces in them.  This change uses Curl's URL-encode API to
solve this.

Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330

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

The absolute udisks:// URI is not usable with InputStream::Open(), and
thus we need to return the LocalStorage::MapUTF8() return value
instead.

Documentation will follow soon.