* ports.h (enum scm_port_rw_active): New enum, containing

* ports.h (enum scm_port_rw_active): New enum, containing
SCM_PORT_READ, SCM_PORT_WRITE, and SCM_PORT_NEITHER (instead of
zero).  The debugger knows about enums, but doesn't know about
#defines.
(typedef scm_port): Declare rw_active member to be an enum
scm_port_rw_active.
* fports.c (fport_flush, fport_end_input): Use SCM_PORT_NEITHER
instead of zero.
* ports.c (scm_add_to_port_table): Same.
* strports.c (st_flush, st_end_input): Same.