Support for non-blocking I/O

* doc/ref/api-io.texi (I/O Extensions): Document read_wait_fd /
  write_wait_fd members.
  (Non-Blocking I/O): New section.
* libguile/fports.c (fport_read, fport_write): Return -1 if the
  operation would block.
  (fport_wait_fd, scm_make_fptob): Add read/write wait-fd
  implementation.
* libguile/ports-internal.h (scm_t_port_type): Add read_wait_fd /
  write_wait_fd.
* libguile/ports.c (default_read_wait_fd, default_write_wait_fd): New
  functions.
  (scm_make_port_type): Initialize default read/write wait fd impls.
  (trampoline_to_c_read, trampoline_to_scm_read)
  (trampoline_to_c_write, trampoline_to_scm_write): To Scheme, a return
  of #f indicates EWOULDBLOCk.
  (scm_set_port_read_wait_fd, scm_set_port_write_wait_fd): New
  functions.
  (port_read_wait_fd, port_write_wait_fd, scm_port_read_wait_fd)
  (scm_port_write_wait_fd, port_poll, scm_port_poll): New functions.
  (scm_i_read_bytes, scm_i_write_bytes): Poll if the read or write would
  block.
* libguile/ports.h (scm_set_port_read_wait_fd)
  (scm_set_port_write_wait_fd): Add declarations.
* module/ice-9/ports.scm: Shunt port-poll and port-{read,write}-wait-fd
  to the internals module.
* module/ice-9/sports.scm (current-write-waiter):
  (current-read-waiter): Implement.
* test-suite/tests/ports.test: Adapt non-blocking test to new behavior.
* NEWS: Add entry.

libguile/ports-internal.h

@@ -44,6 +44,9 @@ struct scm_t_port_type
   SCM scm_read;
   SCM scm_write;
 
+  int (*read_wait_fd) (SCM port);
+  int (*write_wait_fd) (SCM port);
+
   scm_t_off (*seek) (SCM port, scm_t_off OFFSET, int WHENCE);
   void (*close) (SCM port);