Use an URN for DTD ID

XML tools interpret “fonts.dtd” as a relative path.

Unfortunately, that can not work now that the configuration is spread over
multiple system-dependant directories, without a common relative path to this
file. And, an absolute path can not be defined in a system-independant way.
System independance is a requirement to share config files between systems.

Therefore, replace the broken relative path by a formal URN, that will work the
same way on all systems, without network access.

This makes the DTD registerable with commands like:
$ xmlcatalog --noout --add system \
                           "urn:fontconfig:fonts.dtd" \
                           "file:///usr/share/xml/fontconfig/fonts.dtd" \
                           /etc/xml/catalog

That enables easy config file checking:
$ xmllint --loaddtd ${config_file} >/dev/null
3 jobs for master in 5 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #2107178
mingw-build

00:04:18

passed #2107176
shared-build

00:05:34

passed #2107177
static-build

00:04:15