Fix Autotools AsciiDoc configuration + CSS updates.

Fix --asciidoc-opts in Makefile.am.

Add borders around menu and keycap markup. I like the latter but the
former could probably use more work.

Change-Id: If02e3d00875106d6b8f8bcf01cd52b1df4f9b6c8
Reviewed-on: https://code.wireshark.org/review/3817
Reviewed-by: Gerald Combs <gerald@wireshark.org>