Fixes regression by commit 4e2a551f

Closes https://github.com/MusicPlayerDaemon/MPD/issues/599

Fix src/ls.cxx to only print unique schemas.
Refactor src/ls.cxx to use src/input/InputPlugin functionality.

Add dynamic enumeration support to curl plugin.

Required according to RFC 3986:

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

Closes #330

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

Simplify the definition of many build options.

I've created an elementary input plugin that plays sound from the
soundcard, so you can use MPD to listen to anything connected to the
line-in jack, or to Video4Linux FM radio cards that send audio through
the soundcard.  There has been a small number of posts here in the
past requesting line-in input, so here is a first, simplistic stab at
it.

The patch adds a new sheme, alsa://, which causes mpd to play data
read directly from a souncdard.  It defaults to hw:0,0, but you can
pass any ALSA device name in the URI.  So, using mpc for example:

 mpc add alsa://
 mpc play

will play from device hw:0,0.

To use a diffferent device:

 mpc add alsa://hw:1,0

Replaces GLib's g_str_has_prefix().

This plugin is cumbersome to support, now that MPD is migrating away
from GLib and the GLib event loop.  It has no practical advantages
over the CURL plugin.  Soup requires the bloated GType library.

Remove GLib dependency from some headers.

To demonstrate the new I/O thread.  libsoup is well-integrated into
the GLib main loop, which made this plugin pretty easy to write.

As a side effect, we have to initialize the I/O thread in all debug
programs that use the input API.

For Spotify tracks. Uses a spt URI, so with mpc you can play tracks
with e.g.,

  mpc add spt://spotify:track:5qENVY0YEdZ7fiuOax70x1
  mpc play

Uses the pcm_decoder_plugin for the output