Fix tzsetup not installing /etc/localtime for UTC

If tzsetup UTC is run then it successfully configured the system for UTC
including installing /etc/localtime however if the user ran just tzsetup
for interactive configuration and select UTC no /etc/localtime was installed
which resulted in failures for utilities which require said file.

Change set_zone_utc to call install_zoneinfo("UTC") to ensure that
/etc/localtime is created for interactive UTC selection.

Users who have previously run tzsetup in interactive mode and select UTC
can install the missing /etc/localtime by running tzsetup -r.

Also correct static miss-match for set_zone_utc.

MFC after:	2 weeks
Relnotes:	Yes
Sponsored by:	Multiplay

usr.sbin/tzsetup/tzsetup.c

@@ -199,6 +199,7 @@ static char *chrootenv = NULL;
 static void	usage(void);
 static int	confirm_zone(const char *filename);
 static int	continent_country_menu(dialogMenuItem *);
+static int	install_zoneinfo(const char *zoneinfo);
 static int	install_zoneinfo_file(const char *zoneinfo_file);
 static int	set_zone_multi(dialogMenuItem *);
 static int	set_zone_whole_country(dialogMenuItem *);
@@ -633,13 +634,13 @@ set_zone_menu(dialogMenuItem *dmi)
 	return (DITEM_LEAVE_MENU);
 }
 
-int
+static int
 set_zone_utc(void)
 {
 	if (!confirm_zone(NULL))
 		return (DITEM_FAILURE | DITEM_RECREATE);
 
-	return (install_zoneinfo_file(NULL));
+	return (install_zoneinfo("UTC"));
 }
 
 static int