maint: Make ctags work out of the box

The .ctags file specifies default options for ctags so that it does not
ignore libvirt.h.in and ignores uninteresting files. As a result, you
can just run "ctags" and navigating to a public API won't get you to a
useless entry in api.html.

.ctags

+--recurse
+--exclude=*.orig
+--exclude=*.html
+--exclude=*.html.in
+--langmap=c:+.h.in