Replace `setbinary' by a public `%default-port-encoding' fluid.

Replace `setbinary' by a public `%default-port-encoding' fluid.

* doc/ref/api-evaluation.texi (Character Encoding of Source Files): Add
  reference to the "Ports" node.

* doc/ref/api-io.texi (Ports): Document `%default-port-encoding'.

* libguile/ports.c (scm_port_encoding_var): Rename to...
  (default_port_encoding_var): ... this; update callers.  Make `static'.

* libguile/posix.c (scm_setbinary): Remove.

* libguile/posix.h: Adjust accordingly.

* test-suite/tests/numbers.test: Remove unneeded `setbinary' call.

* test-suite/tests/ports.test: Replace `setbinary' call by equivalent
  `%default-port-encoding' mutation and `set-port-encoding!' calls.

* test-suite/tests/r6rs-ports.test: Replace `setbinary' call by
  equivalent `%default-port-encoding' mutation.