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.

Refactor SimpleDatabase::GetLastModified() to be generic for all
plugins.  Remove the SimpleDatabase assumption from db_stats_print(),
allowing it to be implemented by all database plugins.

Fixes crash on "stats" in certain configurations.

Eliminate redundant declaration.

Unused outside of Stats.cxx.

If no music_directory is configured, return nullptr instead of an
empty string.  This fixes a crash when db_file is configured without
music_directory.

Fixes build tests on ia64 and mipsel by testing for endianness in a
more general / portable way.