Intuitionize fndmin in iset.mm
Intuitionize fndmin in iset.mm

This is just a matter of changing dffn5 to dffn5im (that is already
the only direction the proof was using) and fvex to funfvex (the
condition needed was already present), and rearranging the
logic accordingly.

metamath/set.mm,metamath/set.mm,metamath/set.mm