Extendend memory is mapped over the main memory in the 0x4000..0x7FFF
area. Many DOSes disable interrupts while extended memory is banked in,
but not all (e.g. SpartaDOS-X).
This change modifies the initial interrupt handler to map in main memory
before chaining to the "worker" handlers.
Since the initial interrupt handler uses a data segment to store the
trampoline to chain to the original handler, introduce a new "LOWBSS"
segment to hold this trampoline. Otherwise the trampoline may end up
inside the 0x4000..0x7FFF area.
Add a link time warning if "LOWCODE" segment lays within the extended
memory window.
The way we want to use the INITBSS segment - and especially the fact that it won't have the type bss on all ROM based targets - means that the name INITBSS is misleading. After all INIT is the best name from my perspective as it serves several purposes and therefore needs a rather generic name.
Unfortunately this means that the current INIT segment needs to be renamed too. Looking for a short (ideally 4 letter) name I came up with ONCE as it contains all code (and data) accessed only once during initialization.
