Missing getImageName() in Unix and Win32 VM support code

In VMMaker 3.8b2 (latest on SM) there are two references to getImageName()
in class Interpreter. The function exists in RiscOS and Mac OS support code,
but not in Unix or Win32 (as checked out from svn earlier today).

Trivial patches for Unix and Win32 are attached (tested on Linux, not on Windows).

Dave