join: improve memory management

join: improve memory management

* src/join.c (struct seq): Use a (struct line **) for `lines' rather than
one long (struct line *).  This allows individual lines to be swapped out
if necessary.
(reset_line): Get a line ready for new input.
(init_linep): Create a new line and assign it to the the pointer passed in.
(spareline[2]): Hold a spare line for each input file.
(free_spareline): Clean up.
(get_line): Take a (struct line **) instead of a (struct line *).  If the
line to be overwritten is the previous line for the current file then swap
it out for the spare.
(join): Accomodate new structure of SEQs and new parameters to get_line;
Don't free stale lines until the end -- they're re-usable now.
(dup_line): Remove function.
* NEWS: Mention the performance improvement.