Peeks do not consume EOFs.

Peeks do not consume EOFs.

Fixes <http://bugs.gnu.org/12216>.

* libguile/ports-internal.h (struct scm_port_internal): Add
  'pending_eof' flag.

* libguile/ports.c (scm_i_set_pending_eof, scm_i_clear_pending_eof): New
  static functions.
  (scm_new_port_table_entry): Initialize 'pending_eof'.
  (scm_i_fill_input): Check for 'pending_eof'.
  (scm_i_peek_byte_or_eof): Set 'pending_eof' flag before returning EOF.
  (scm_end_input, scm_unget_byte, scm_seek, scm_truncate): Clear
  'pending_eof'.
  (scm_peek_char): Set 'pending_eof' flag before returning EOF.

* test-suite/tests/ports.test ("pending EOF behavior"): Add tests.