Add missing R6RS `open-file-input/output-port' procedure

Add missing R6RS `open-file-input/output-port' procedure

* module/rnrs/io/port.scm (r6rs-open): New internal helper procedure for
  opening files.
  (open-file-input-port, open-file-output-port): Make use of
  `r6rs-open'.
  (open-file-input/output-port): Implement in terms of `r6rs-open',
  add to exported identifiers list.

* module/rnrs.scm (open-file-input/output-port): Add to exported
  identifiers.

* test-suite/tests/r6rs-ports.test (test-input-file-opener): New
  procedure, collects several tests for opening file input ports.
  ("7.2.7 Input Ports"): Use `test-input-file-opener' for checking
  `open-file-input-port'.
  (test-output-file-opener): New procedure, collects several tests for
  opening file output ports.
  ("8.2.10 Output ports"): Use `test-output-file-opener' for checking
  `open-file-output-port'.
  ("8.2.13 Input/output ports"): New test prefix, making use of both
  `test-input-file-opener' and `test-output-file-opener' to check
  `open-file-input/output-port'.