Prepare for a ICU-based backend without GLib.

Don't expose pointer that requires the caller to invoke g_free(),
because that's GLib-only.

Don't abort the process, let the caller decide instead.

Simplify the definition of many build options.

MPD_PATH_MAX_UTF8 is only used by GLib-specific code currently.

Necessary for "final" on gcc 4.6.

Several MPD subsystems can now read gzipped files; for example, the
database file can be gzipped.

The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

Support operating systems that don't have O_CLOEXEC.

Eliminate the std::string overhead.

The XDG cache directory is part of the "base directories" spec like
$XDG_CONFIG_HOME, not "user directories".

Move code from CreateConfiguredDatabase() and add XDG support.  This
implements an automatic Linux fallback for the setting "db_file" if
none was specified.

The upper-case name seems to be the canonical one.