Fix startup crashes caused by wrong memory map handling

init_x86_vm() takes the memory map from BIOS and EFI and selects
regions suitable for memory allocation. This involves removing
areas used by the kernel, but the logic missed some corner cases,
which led to possible allocation in regions for which later memory
access would cause a panic.

The typical panic from this bug in GENERIC is at SVS startup:
cpu_svs_init / uvm_pagealloc_strat / pagezero

We fix the bug by adding logic for the missing cases of memory
regions overlapping with the kernel. While there, add more #idef'ed
debug output.

(manu)