Make crypt and crypt_gensalt use thread-local output buffers.

This change makes crypt and crypt_gensalt as thread-safe as
they can be without changing their interfaces.  Solaris
already made this change, and it’s being discussed by glibc
(with suggestion that it should be pushed upstream to the C
and POSIX standards committees):
https://sourceware.org/ml/libc-alpha/2018-10/msg00437.html

Portable programs should still use the r-variants, though,
because this is not a guaranteed feature, it doesn’t make
them not clobber their output buffers on a second call from
within the same thread, and the tradeoff is a some overhead
for initializing a buffer of `sizeof (crypt_data)` and for
copying the computed output to the thread-local output buffer
for each invocation of the crypt function.

lib/crypt-static.c

@@ -25,8 +25,17 @@
 char *
 crypt (const char *key, const char *setting)
 {
-  static TLS_LD struct crypt_data nr_crypt_ctx;
-  return crypt_r (key, setting, &nr_crypt_ctx);
+  struct crypt_data nr_crypt_ctx;
+  static TLS_LD char output[CRYPT_OUTPUT_SIZE];
+
+  crypt_r (key, setting, &nr_crypt_ctx);
+  strcpy_or_abort (output, sizeof (output), nr_crypt_ctx.output);
+
+#if ENABLE_FAILURE_TOKENS
+  return output;
+#else
+  return output[0] == '*' ? 0 : output;
+#endif /* ENABLE_FAILURE_TOKENS */
 }
 SYMVER_crypt;
 #endif