Arrange to call net_pending_errors on Unix, which we've never actually

remembered to do before! Also some related fixes, such as that after
we do so we should immediately stop selecting on the socket in
question.

[originally from svn r9363]

unix/uxsftp.c

@@ -605,6 +605,8 @@ char *ssh_sftp_get_cmdline(char *prompt, int no_fds_ok)
     }
 }
 
+void frontend_net_error_pending(void) {}
+
 /*
  * Main program: do platform-specific initialisation and then call
  * psftp_main().