mdb_env_set_flags(): Reject CHANGELESS flags.

Reverts part of a2ac10107e.
MDB_ROBUST needed to be accepted, but that flag is gone.

libraries/liblmdb/lmdb.h

@@ -736,7 +736,6 @@ void mdb_env_close(MDB_env *env);
 	 * This may be used to set some flags in addition to those from
 	 * #mdb_env_open(), or to unset these flags.  If several threads
 	 * change the flags at the same time, the result is undefined.
-	 * Most flags cannot be changed after #mdb_env_open().
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] flags The flags to change, bitwise OR'ed together
 	 * @param[in] onoff A non-zero value sets the flags, zero clears them.

libraries/liblmdb/mdb.c

@@ -9001,7 +9001,7 @@ mdb_env_copy(MDB_env *env, const char *path)
 int ESECT
 mdb_env_set_flags(MDB_env *env, unsigned int flag, int onoff)
 {
-	if (flag & (env->me_map ? ~CHANGEABLE : ~(CHANGEABLE|CHANGELESS)))
+	if ((flag & CHANGEABLE) != flag)
 		return EINVAL;
 	if (onoff)
 		env->me_flags |= flag;