Users now have to call Close() explicitly.  This simplifies using the
class, as most users have automatic socket management already, and
Steal() had to be used often.

Fixes a crash bug.  See code comment.

Migrate from the old curl_multi_perform() API to the newer
curl_multi_socket_action() API (since CURL 7.16).

This allows working around a bug with HTTP redirections with epoll:
when CURL closes a socket and the new one happens to have the same
file number, MPD did not have a chance to remove the old one from
epoll and subsequently attempted to use EPOLL_CTL_MOD, which was not
allowed by epoll, because it's a new socket now.

Move all global variables there, and keep just one global variable:
the pointer to the CurlMulti instance.  Prepares for the next commit.

Unused.

Eliminate some complicated code that's not worth the trouble.

Replaces the loop in input_curl_find_request().

Replaces GLib's g_ascii_strcasecmp().

Automatic memory management.

Decouple some more from GLib.

Micro-optimization.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

When playing finite http streams, e.g. something from soundcloud,
eventloop stalls after pausing writefunc. TimeoutMonitor is cancelled
by the time resume happens, so when enough data is consumed writefunc
is called only once. Calling InvalidateSockets() from
input_curl_resume() seems to fix the issue.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Handle timeout internally.

Simplify the API, don't use GLib specific integer type.

Replaces io_thread_call().  This approach is more generic and easier
to use due to std::function.