Add system.birthdate_template string property

Suitable for passing to console.gettemplate() when prompting for a birth date.

I'm using like this in my logon.js to correct corrupted newuser birth dates:
  while(user.age < 10) {
	console.putmsg(bbs.text(bbs.text.EnterYourBirthday));
	user.birthdate = console.gettemplate(system.birthdate_template);
  }

src/sbbs3/js_system.c

@@ -52,6 +52,7 @@ enum {
 	, SYS_PROP_DATE_SEP
 	, SYS_PROP_DATE_VERBAL
 	, SYS_PROP_BIRTHDATE_FMT
+	, SYS_PROP_BIRTHDATE_TEMPLATE
 	, SYS_PROP_PWDAYS
 	, SYS_PROP_MINPWLEN
 	, SYS_PROP_MAXPWLEN
@@ -190,6 +191,10 @@ static JSBool js_system_get(JSContext *cx, JSObject *obj, jsid id, jsval *vp)
 			birthdate_format(cfg, str, sizeof str);
 			p = str;
 			break;
+		case SYS_PROP_BIRTHDATE_TEMPLATE:
+			birthdate_template(cfg, str, sizeof str);
+			p = str;
+			break;
 		case SYS_PROP_NODES:
 			*vp = INT_TO_JSVAL(cfg->sys_nodes);
 			break;
@@ -460,6 +465,8 @@ static jsSyncPropertySpec js_system_properties[] = {
 		, JSDOCSTR("Short date month-name displayed verbally instead of numerically")},
 	{   "birthdate_format",         SYS_PROP_BIRTHDATE_FMT, SYSOBJ_FLAGS,   32002
 		, JSDOCSTR("User birth date input and display format (MM=Month number, DD=Day of month, YYYY=Year)")},
+	{   "birthdate_template",       SYS_PROP_BIRTHDATE_TEMPLATE, SYSOBJ_FLAGS, 32002
+		, JSDOCSTR("User birth date input template")},
 	{   "pwdays",                   SYS_PROP_PWDAYS,    SYSOBJ_FLAGS,       310
 		, JSDOCSTR("Days between forced user password changes (<tt>0</tt>=<i>never</i>)")},
 	{   "min_password_length",      SYS_PROP_MINPWLEN,  SYSOBJ_FLAGS,       31702