catopen: set errno to EOPNOTSUPP

Per 1003.1-2008 (2016 ed.), catopen must set errno on failure.

We set errno to EOPNOTSUPP because musl does not currently support
message catalogues.

src/locale/catopen.c

@@ -1,6 +1,8 @@
 #include <nl_types.h>
+#include <errno.h>
 
 nl_catd catopen (const char *name, int oflag)
 {
+	errno = EOPNOTSUPP;
 	return (nl_catd)-1;
 }