Finds a given file in the VAT, if it exists, searching based on the contents of the file. This routine is designed to be reentrant, in the sense that you can call it multiple times with inputs based on its outputs. For example, you can call it the first time with hl=(progptr), and then subsequent times with hl = the de output of the previous run. It can also properly deal with archived programs. This is particularly useful for finding game levels that always have a specific header (or even DCS APs!).
hl = Pointer to where to begin in the VAT, for example (progptr)
ix = Detection string, zero-terminated
de = Address of place stopped in the VAT + 1
hl = Pointer to first byte of program data
z flag = set if no program was found (hl and de are invalid), and reset if a program was found
af, bc, de, hl