xmlconfig: read more config files from drirc.d/

Driver and application can put their drirc files in
${datadir}/drirc.d/ with name xxx.conf. Config files
will be read and applied in file name alphabetic order.

So there are three places for drirc listed in order:
1. /usr/share/drirc.d/
2. /etc/drirc
3. ~/.drirc

v4:
  fix meson build

v3:
  1. seperate driParseConfigFiles refine into another patch
  2. fix entries[i] mem leak

v2:
  drop /etc/drirc.d

Signed-off-by: Qiang Yu <Qiang.Yu@amd.com>
Acked-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

docs/autoconf.html

@@ -94,6 +94,13 @@ Currently there's only one config file provided when dri drivers are
 enabled - it's <code>drirc</code>.</p>
 </dd>
 
+<dt><code>--datadir=DIR</code></dt>
+<dd><p>This option specifies the directory where the data files will
+be installed. The default is <code>${prefix}/share</code>.
+Currently when dri drivers are enabled, <code>drirc.d/</code> is at
+this place.</p>
+</dd>
+
 <dt><code>--enable-static, --disable-shared</code></dt>
 <dd><p>By default, Mesa
 will build shared libraries. Either of these options will force static