dyninstAPI: Let image::findMain inspect PIE too

dyninstAPI: Let image::findMain inspect PIE too

Position-independent executables (PIE) are generally indistinguishable
from any other shared object, but we still need to find main() in these
when they're stripped.  One clue is the presence of INTERP, which PIEs
will always have.  (Some DSOs also have INTERP, but this is rare.)