Change `make-custom-binary-input-port' to require 5 arguments.

Change `make-custom-binary-input-port' to require 5 arguments.

* src/ports.c (scm_r6rs_make_custom_binary_input_port): Have 5 required
  arguments, instead of 2 + 3 optional, which did not conform to the
  standard.  Handle `#f' arguments.

* doc/api-r6rs.texi (R6RS Binary Input): Update description of
  `make-custom-binary-input-port'.

* tests/io-ports.test ("7.2.7 Input
  Ports")["make-custom-binary-input-port [wrong-num-args]"]: New test.
  Update callers of `make-custom-binary-input-port'.