* fports.c (scm_fdes_to_port): call fcntl F_GETFL to test that

	* fports.c (scm_fdes_to_port): call fcntl F_GETFL to test that
	the fdes is valid before doing anything else.  check that
	the file descriptor supports the modes required.
	(scm_fport_buffer_add): don't throw an error if fstat doesn't
	work: just use the default buffer size.

	* throw.c: change an outdated comment about scm_internal_catch
	BODY: it doesn't take a jumpbuf arg.

	* init.c (scm_standard_stream_to_port): install a handler in case
	scm_fdes_to_port throws an error.  don't check here whether the
	file descriptor is valid, since scm_fdes_to_port will do that.
	set the revealed count depending on whether the port got the
	standard file descriptor.
	(stream_body_data): new type.
	(stream_body, stream_handler): new procs.