PR bin/50993 - this is a significant rewrite of the way that here
documents are processed.  Now, when first detected, they are
simply read (the only change made to the text is to join lines
ended with a \ to the subsequent line, otherwise end marker detection
does not work correctly (for here docs with an unquoted endmarker
only of course.)  This patch also moves the "internal subroutine"
for looking for the end marker out of readtoken1() (which had to
happen as readtoken1 is no longer reading the here doc when it is
needed) - that uses code mostly taken from FreeBSD's sh (thanks!)
and along the way results in some restrictions on what the end
marker can be being removed.  We still do not allow all we should.
(from kre@)

(christos)