Make the DOSVER function call return 6Fh in C in DOS 1 mode

[Konamiman]

MSX-DOS doesn't modify the main registers when an unknown function call is invoked, and the Dutch Moonsound Veterans music disk uses this fact to detect the MSX-DOS version in use, like this:

ld c,6Fh  ;_DOSVER function (introduced in MSX-DOS 2)
call F37Dh
ld a,c
cp 6Fh
ret z
;DOS 2 assumed from this point

However this MSX-DOS version detection method is technically incorrect. The MSX-DOS 2 function codes specification says the following regarding the _DOSVER function (emphasis mine):

For compatibility with MSX-DOS 1.0, the following procedure should always be followed in using this function. Firstly if there is any error (A<>0) then it is not MSX-DOS at all. Next look at register B. If this is less than 2 then the system is earlier than 2.00 and registers C and DE are undefined.

Nextor does corrupt the C register when _DOSVER is invoked in MSX-DOS 1 mode (unless the magic numbers are supplied, see the relevant documentation) and this is what causes the music disk to fail. So strictly speaking the reported issue is not a bug in Nextor but an incorrect usage of the function.

However, the fix is trivial and makes Nextor work more consistently with how "pure" MSX-DOS 1 does, thus this pull request implements it: now when _DOSVER is invoked in MSX-DOS 1 mode without the magic numbers supplied, it will preserve the contents of register C.

Closes #120.