Prepare 'setvbuf' to support for non-file ports.

Prepare 'setvbuf' to support for non-file ports.

* libguile/ports-internal.h (struct scm_port_internal): Add
  setvbuf' field.  Change 'pending_eof' to a 1-bit unsigned char.
* libguile/ports.c (scm_new_port_table_entry): Clear 'pti->setvbuf'.
* libguile/fports.c (scm_setvbuf): Accept any open port, and error out
  when PORT's setvbuf' field is NULL.  Remove explicit 'scm_gc_free' calls.
  (scm_i_fdes_to_port): Set PORT's 'setvbuf' field.
* test-suite/tests/ports.test ("setvbuf")["closed port", "string port"]:
  New tests.
* doc/ref/posix.texi (Ports and File Descriptors): Suggest that
  'setvbuf' works for different port types.