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.

Eliminate duplicate code.

Allows constructing the request first and set the URL later.  This is
needed because curl_easy_escape() is needed to construct the URL,
which however needs the CURL "easy" handle created by the Request class.

Bugs found by Coverity.

Needed for WStringView.

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

libcurl's WRITEFUNCTION is pretty fragile; if we destroy the CURL*
instance or even unregister it using curl_multi_remove_handle(),
libcurl will crash instantly.  But still we need to be able to handle
exceptions from inside the WRITEFUNCTION, and call
CurlResponseHandler::OnError(), which may destroy the whole thing.  As
a workaround, I use DeferredMonitor to postpone the OnError() call
into a stack frame which is allowed to destroy the request.

Let the caller decide what to do with the exception.