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.

This variable is initialized once on startup.  It will never change.

Reduce GLib usage.

Don't use GTimer if the operating system is able to tell us the
uptime.

This implementation behaves mostly identical to old parser.
Few observable differences:
- There are no option groups (single group is used for all options)
- Option --stdout is hidden (it has been obsolete for a long time)
- MPD executable name (mpd) is hardcoded for simplicity

Using pcm_resample_lsr_32() for 24 bit samples works, but may cause 24
bit overflows.  This commit makes 24 bit a special case with explicit
clipping.