Convert AForms field types to class hierarchy

Once-unrelated classes are now a fully-featured class hierarchy,
featuring (emulated, due to PHP’s limitations) multiple and
diamond inheritance.
Add setuid helper for SFUMA file operations

SFUMA now uses a setuid binary to overwrite files the HTTP server
might not have permission to touch.
Fix JS accessing members of inaccessible chrome elements
Expand sfuma-sudo to handle more commands (cp, ct)
Add test suite to sfuma-sudo’s Makefile

Now necessary, due to expanded feature set.