POSIX Asynchronous I/O support: fhandler files

This code is where the AIO implementation is wired into existing Cygwin
mechanisms for file and device I/O: the fhandler* functions.  It makes
use of an existing internal routine prw_open to supply a "shadow fd"
that permits asynchronous operations on a file the user app accesses
via its own fd.  This allows AIO to read or write at arbitrary locations
within a file without disturbing the app's file pointer.  (This was
already the case with normal pread|pwrite; we're just adding "async"
to the mix.)

winsup/cygwin/fhandler_tty.cc

@@ -859,7 +859,7 @@ fhandler_pty_slave::read (void *ptr, size_t& len)
 	break;
     }
 out:
-  termios_printf ("%d=read(%p, %lu)", totalread, ptr, len);
+  termios_printf ("%d = read(%p, %lu)", totalread, ptr, len);
   len = (size_t) totalread;
 }