MXS-1354: Add JSON serialization of users

The users can now be dumped and loaded as JSON objects. This allows easier
parsing and handling of users while still retaining the possibility to
manually edit the output. Added tests for dumping and loading the JSON
form users.

Also fixed a deadlock in Users::remove() where the same lock was acquired
twice and a faulty test case where failed authentication was expected to
work.

include/maxscale/users.h

@@ -21,6 +21,7 @@
 #include <maxscale/dcb.h>
 #include <maxscale/listener.h>
 #include <maxscale/service.h>
+#include <maxscale/jansson.h>
 #include <openssl/sha.h>
 
 MXS_BEGIN_DECLS
@@ -28,6 +29,7 @@ MXS_BEGIN_DECLS
 /** User account types */
 enum account_type
 {
+    ACCOUNT_UNKNOWN,
     ACCOUNT_BASIC, /**< Allows read-only access */
     ACCOUNT_ADMIN  /**< Allows complete access */
 };
@@ -126,6 +128,26 @@ bool users_promote(USERS* users, const char* user);
  */
 bool users_demote(USERS* users, const char* user);
 
+/**
+ * Dump users as JSON
+ *
+ * The resulting JSON can be loaded later to restore the users.
+ *
+ * @param users Users to dump
+ *
+ * @return JSON form of the users that can be used for serialization
+ */
+json_t* users_to_json(USERS *users);
+
+/**
+ * Load users from JSON
+ *
+ * @param json JSON to load
+ *
+ * @return The loaded users
+ */
+USERS* users_from_json(json_t* json);
+
 /**
  * @brief Default user loading function
  *