[RESEND] mountmgr.sys: implement forced setting of fixed/local disk

[Ben Klein]

define_unix_drive in mountmgr.c includes all cases used in query_unix_drive
except the DRIVE_FIXED == DEVICE_HARDDISK_VOL mapping. This patch fixes 17618
where setting the drive type to "Local disk" in winecfg does not save the
change to the registry.

I've CC'd wine-devel in the hope of getting some feedback this time.
This is the 3rd or 4th time I've submitted this patch.