add scm_c_make_port; the port table is now a weak set

add scm_c_make_port; the port table is now a weak set

* libguile/ports.c (scm_c_make_port_with_encoding, scm_c_make_port): New
  functions, to replace scm_new_port_table_entry.  Use a weak set
  instead of a weak table.
  (scm_i_remove_port):
  (scm_c_port_for_each, scm_port_for_each): Adapt to use weak set.
  (scm_i_void_port): Use scm_c_make_port.
  (scm_init_ports): Make a weak set.

* libguile/fports.c:
* libguile/ioext.c:
* libguile/r6rs-ports.c:
* libguile/strports.c:
* libguile/vports.c: Adapt to use the new scm_c_make_port API.