join: avoid new leaks

join: avoid new leaks

* src/join.c (prevline): Make prevline module-level static, so that
the allocated items can be freed at exit.
(free_prevline): new atexit function; frees items in prevline[].
Use ARRAY_CARDINALITY, so include "argmatch.h" to get that.
(main): Arrange for free_prevline to be called during exit.
(get_line): Free prevline[which - 1] also, as that
will have been allocated by dup_line.