Finish converting the User's Guide to AsciiDoc.

As with the Developer's Guide, add user-guide-docinfo.xml, which lets us
carry over DocBook front matter. Remove the meta_info chapter. Most of
its contents are now in user-guide-docinfo.xml. Add a DocBook revision
history based on hints from the Git/SVN/CVS revision history.

Remove the various makefile targets for converting AsciiDoc files to
DocBook chapters. Remove GPL_appendix.xml. We use the AsciiDoc version.

Change-Id: I543fa2a92f2c735c5f00c97ec65cff2187e09e3a
Reviewed-on: https://code.wireshark.org/review/5216
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>