Add scm_t_dynwind_flags casts

Add scm_t_dynwind_flags casts

There is a handful of calls to

  scm_dynwind_begin (0);

where the parameter is an enum, scm_t_dynwind_flags.  In C++, we have no
choice but to add an explicit cast, since there is no enum value that
represents 0 (no flags set).

gdb/ChangeLog:

	* guile/scm-breakpoint.c (gdbscm_set_breakpoint_stop_x): Add
	scm_t_dynwind_flags casts.
	* guile/scm-cmd.c (gdbscm_parse_command_name): Likewise.
	* guile/scm-ports.c (gdbscm_open_memory): Likewise.
	* guile/scm-value.c (gdbscm_value_to_string): Likewise.