#19 Make rom2_sw return the original RAM page

[Brian Ruthven]

There are only 2 callers of rom2_sw: plus3check() and dos_springboard().

plus3check() doesn't matter.
dos_springboard() currently starts by doing this:

_dos_springboard::
        call    _get_bankm_ram_page     ; Get current RAM page
        ex      de, hl                  ; Save in de (hl is needed)

        call    _rom2_sw                ; Also selects RAM page 7

We could avoid the explicit call to _get_bankm_ram_page by making rom2_sw return the current RAM page prior to switching in RAM page 7. We can simply return the current value of BANKM and pass this later to ram_sw, which will strip any non-RAM-page bits out.

[Brian Ruthven]

Correction, 3 callers, but dos_header() isn't used anywhere (and probably should be deleted anyway).

[Brian Ruthven]

Saves two bytes.