Unescape the base path and the path coming from the server (href) to fix the
comparison when the server uses different escaped characters.

The outputted name need to be unescaped. Doing that before or after the
HrefToEscapedName() call should not change the current behavior.

If the file name is "Hello & bye", 3 CharacterData events will be sent with the
State::HREF state:
 - "Hello%20"
 - "&"
 - "%20bye"

Reproduced with files hosted on an apache2 DAV server: 2.4.38-3+deb10u3.

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

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

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.

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

Bugs found by Coverity.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.