memdisk: additional cleanups

memdisk: additional cleanups

Additional stylistic cleanups.  Rename "syscall" to "intcall" (we
can't call it __intcall without clashing with com32.h); use macros
instead of copying variables to different places with only the type
being different.

Also, only change the rm/pm jump instructions when actually relocating
the code.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>