CMake: More a2x / asciidoc fixes.

Create graphics directories and copy files into them similar
to Autotools and Nmake. Adjust some xsltproc arguments. Fix the
--asciidoc-opts flag.

Note: Admon graphics are broken pending change 3206.

Change-Id: I94d498de36150a7cb4ffd080581523300b222bd4
Reviewed-on: https://code.wireshark.org/review/3805
Reviewed-by: Gerald Combs <gerald@wireshark.org>