segment: Refine treatment of start of macro body.

Previously, if the first line of the macro body (the same line as the
closing parenthesis in the DEFINE) was blank, we reported it as a blank
line to the lexer.  The parser for DEFINE could check for that (by seeing
whether the first line of macro body was empty or all-spaces) but it seems
more elegant to do it in the segmenter.  This implements that.