Sync xdvipdfmx with TeX Live 2012.

Patches provided by Matthias Drochner and modified by me.

Changes:
        * src/spc_pdfm.c: use kpse to find media.
        * dvipdfmx.c: allow inclusion of subsidiary config files, directive i
        * data/config/dvipdfmx.cfg: document the i directive
        * pdfdev.c (handle_multibyte_string): Handle UTP-16 surrogates
        for UCS-4 supplementary planes.

(minskim)