Convert VT102ID to a config variable

VT102ID is the sequence that the terminal returns when it is inquired
to identify itself. This value should be configurable in the same
way that another st parameters.

config.def.h

@@ -9,6 +9,9 @@ static char font[] = "Liberation Mono:pixelsize=12:antialias=false:autohint=fals
 static int borderpx = 2;
 static char shell[] = "/bin/sh";
 
+/* identification sequence returned in DA and DECID */
+static char vtiden[] = "\033[?6c";
+
 /* Kerning / character bounding-box multipliers */
 static float cwscale = 1.0;
 static float chscale = 1.0;