? in identifiers turns out to be used in the field even in non-TASM
mode. Resolve this by allowing it in an identifier still, but treat
'?' by itself the same as we would a keyword, meaning that it needs to
be separated from other identifier characters.
In other words:
a ? b : c ; conditional expression
a?b:c ; seg:off expression seg = a?b, off = c
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>