MINOR: muxes: Add exit status for errors about not implemented features

The MUX_ES_NOTIMPL_ERR exit status is added to allow the multiplexers to
report errors about not implemented features. This will be used by the H1
mux to return 501-not-implemented errors.

include/haproxy/connection-t.h

@@ -323,6 +323,7 @@ enum mux_exit_status {
 	MUX_ES_SUCCESS,      /* Success */
 	MUX_ES_INVALID_ERR,  /* invalid input */
 	MUX_ES_TOUT_ERR,     /* timeout */
+	MUX_ES_NOTIMPL_ERR,  /* not-implemented error */
 	MUX_ES_INTERNAL_ERR, /* internal error */
 	MUX_ES_UNKNOWN       /* unknown status (must be the last) */
 };

src/log.c

@@ -2168,6 +2168,10 @@ int sess_build_logline(struct session *sess, struct stream *s, char *dst, size_t
 				status = 408;
 				s_flags = SF_ERR_CLITO | SF_FINST_R;
 				break;
+			case MUX_ES_NOTIMPL_ERR:
+				status = 501;
+				s_flags = SF_ERR_PRXCOND | SF_FINST_R;
+				break;
 			case MUX_ES_INTERNAL_ERR:
 				status = 500;
 				s_flags = SF_ERR_INTERNAL | SF_FINST_R;