Start converting the User's Guide to AsciiDoc.

Convert the preface and introduction. Update various makefiles.

Change-Id: Id998cabadb6de458f04b7a43bdba4f98e4edb052
Reviewed-on: https://code.wireshark.org/review/3139
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Tested-by: Gerald Combs <gerald@wireshark.org>