Fix builder/parser so it keeps some whitespace on split lines.

If previous line ends with whitespace, or next line starts with whitepsace
we need to preserve a space otherwise things line:
RUN echo\
...hello    (. == space)
will appear as: RUN echohello

Noticed this while looking at #5744 because he had lines ending in &&\

Signed-off-by: Doug Davis <dug@us.ibm.com>
Ah, that makes sense.  The trailing whitespace is much less interesting
than the in-line whitespace though, which is significant way more often. :)