The upcoming .while request will have to re-execute roff(7) lines

The upcoming .while request will have to re-execute roff(7) lines
parsed earlier, so they will have to be saved for reuse - but the
read.c preparser does not know yet whether a line contains a .while
request before passing it to the roff parser.  To cope with that,
save all parsed lines for now.  Even shortens the code by 20 lines.