Install loader.scm and installed-modules.scm into sysconfdir/etc.

They are changed by uim-module-manager program, so they should be
outside PREFIX/share.
It should be reversed order, i.e. $(sysconfdir)/uim should be put before $(datadir)/uim.
Or with upgrade installation, old files in $(datadir)/uim may be used instead of new created files in $(sysconfidr)/uim.