Make port properties accessible from Scheme.

Make port properties accessible from Scheme.

* libguile/ports.c (scm_i_port_alist, scm_i_set_port_alist_x): Removed.
  (scm_i_port_property, scm_i_set_port_property_x): New procedures,
  available from Scheme as '%port-property' and '%set-port-property!'.

* libguile/ports.h (scm_i_port_alist, scm_i_set_port_alist_x): Removed.
  (scm_i_port_property, scm_i_set_port_property_x): New prototypes.

* libguile/read.c (set_port_read_option, init_read_options): Adapt to
  use scm_i_port_property and scm_i_set_port_property_x.