MPS: is_mps is gone

MPS: is_mps is gone

[Gerd Möllmann]

I've just made a pretty basic change, so if you encounter new problems
it could be this:

igc.c used to have a function is_mps that returned true if an address
was in MPS. But, this was an ugly approximationm - it could return false
positives because the MPS API functioh for that was too expensive for
where is_mps was used.

I've now replaced this with a function has_header which returns true if
a client object has an igc_header, which is considered to be the case if
it isn't a built-in object in the data segment. And telling if something
is a built-in object is exact, no false positives.