vmd diskfmt regress: don't copy source files, cleanup disk images

vmd diskfmt regress: don't copy source files, cleanup disk images

Previous version of the diskfmt regress tests for vmd(8) would copy
source files from the tree in order to compile them. Using some of
the ssh regress tests as inspiration, use the .PATH to allow building
object files from other parts of the tree in place.

Since 4G disk images are made as part of the test, clean those up
when done by adding in a proper REGRESS_CLEANUP target.

"go for it" mlarkin@