Add an update-cache mode to pango-query-modules

Add an update-cache mode to pango-query-modules

It is inconvenient for distro-builders to redirect the output
of pango-query-modules to a location that depends on various
factors, such as architecture and pango module version.

With this commit, you can just run pango-querymodules --update-cache,
and it will replace the file #libdir/pango/1.8.0/modules.cache.