Make utmpx(3) thread safe if we support TLS.

Because the utmpx interface is generally not required to be thread-safe,
but it is nice to have, if easy to do so. Therefore don't make a mess
out of the code and only use it if __NO_TLS is not defined.

lib/libc/gen/getutxent.c

@@ -38,8 +38,13 @@ __FBSDID("$FreeBSD$");
 #include "utxdb.h"
 #include "un-namespace.h"
 
+#ifdef __NO_TLS
 static FILE *uf = NULL;
 static int udb;
+#else
+static _Thread_local FILE *uf = NULL;
+static _Thread_local int udb;
+#endif
 
 int
 setutxdb(int db, const char *file)

lib/libc/gen/utxdb.c

@@ -126,7 +126,11 @@ utx_to_futx(const struct utmpx *ut, struct futx *fu)
 struct utmpx *
 futx_to_utx(const struct futx *fu)
 {
+#ifdef __NO_TLS
 	static struct utmpx *ut;
+#else
+	static _Thread_local struct utmpx *ut;
+#endif
 
 	if (ut == NULL) {
 		ut = calloc(1, sizeof *ut);