Prepare for adding more resamplers.

The PcmResampler interface is implemented by the two classes
FallbackPcmResampler and LibsampleratePcmResampler.  This prepares for
adding more resampler libraries.

Don't use src_format.  In the middle of Convert(), the current
AudioFormat has already been modified, it's now something in between
src_format and dest_format.  This simplifies keeping track of what
remains to be done.

Wrapper for pcm_resample_global_init(), just in case other PCM
libraries need initialization, too.

All parameters seem mandatory, otherwise this call makes MPD crash.

Rename HAVE_WINDOWS to host_is_windows for consistency.
Use 'yes' as true value instead of '1' for this variable.
Use test on this variable instead of case where applicable.

Remove compatibility code.

Replaces GLib's g_str_has_prefix().

Method PcmConvert::Convert() modifies the src_format variable.  This
used to be a parameter, however commit d2679f59 made it an attribute
instead.  The modification to src_format persisted, and the next call
would return garbage.

This change adds two configuration options:

  --with-eventloop=[glib|internal|auto]
  --with-pollmethod=[epoll|auto]

First allows switching between GLib event loop and internal one.
Second chooses backend to use for internal event loop.
Conditional compilation symbols are changed accordingly.
Additional helper macro MPD_OPTIONAL_FUNC_NODEF is added as well.

Otherwise sockets can't be created during IO thread initialization.

Hard-code the program name to "mpd" instead of using g_get_prgname().
The latter has become useless since this GLib variable doesn't get
initialized anymore.