This format has several advantages:
* The CONFIG block need not be moved.
* The entire compressed image (depacker and payload) need
not be moved another time before the UPX depacker's own
operation.
* The CONFIG block always lives at 00602h, and the kernel
need not be aware whether it was compressed for detecting
which CONFIG block to use.
* Support for compressed images beyond 64 KiB for free.
(The assembly define TEST_FILL_INIT_TEXT can be passed in
NASMENV to test this support with 32 KiB of LFSR output.)
* A subsequent commit will shorten the stub to 64 bytes,
compared to the prior 32 + 45 = 77 bytes, with no loss
of features. (The packed payload is a bit shorter too.)
* The new stub also sets ds and es to the segment value
that would point to the DOS/EXE process's PSP. This is
apparently not used by the UPX depacker but could be in
a future or past version, or if another packer is used.
