Add wpa_msg_ctrl() for ctrl_interface-only messages

Add wpa_msg_ctrl() for ctrl_interface-only messages

This is like wpa_msg(), but the output is directed only to
ctrl_interface listeners. In other words, the output will not be
shown on stdout or in syslog.

Change scan result reporting to use wpa_msg_ctrl() for
CTRL-EVENT-SCAN-RESULTS message at info level and wpa_printf() at
debug level to avoid showing scan result events in syslog in the
common configuration used with NetworkManager.