MXS-1220: Add execution of module commands to REST API

The module command self links now point to an endpoint that executes the
module command. Depending on the type of the module command, either a GET
or a POST request must be made.

include/maxscale/modulecmd.h

@@ -65,6 +65,13 @@ typedef struct
                                           This should always be the first argument
                                           if the function requires an output DCB. */
 
+/** What type of an action does the command perform? */
+enum modulecmd_type
+{
+    MODULECMD_TYPE_PASSIVE, /**< Command only displays data */
+    MODULECMD_TYPE_ACTIVE   /**< Command can modify data */
+};
+
 /**
  * Options for arguments, bits 9 through 32
  */
@@ -123,6 +130,7 @@ typedef struct modulecmd
 {
     char                 *identifier; /**< Unique identifier */
     char                 *domain; /**< Command domain */
+    enum modulecmd_type   type; /**< Command type, either active or passive */
     MODULECMDFN           func; /**< The registered function */
     int                   arg_count_min; /**< Minimum number of arguments */
     int                   arg_count_max; /**< Maximum number of arguments */
@@ -130,6 +138,9 @@ typedef struct modulecmd
     struct modulecmd     *next; /**< Next command */
 } MODULECMD;
 
+/** Check if the module command can modify the data/state of the module */
+#define MODULECMD_MODIFIES_DATA(t) (t->type == MODULECMD_TYPE_ACTIVE)
+
 /**
  * @brief Register a new command
  *
@@ -143,7 +154,8 @@ typedef struct modulecmd
  * @return True if the module was successfully registered, false on error
  */
 bool modulecmd_register_command(const char *domain, const char *identifier,
-                                MODULECMDFN entry_point, int argc, modulecmd_arg_type_t *argv);
+                                enum modulecmd_type type, MODULECMDFN entry_point,
+                                int argc, modulecmd_arg_type_t *argv);
 
 /**
  * @brief Find a registered command
@@ -196,6 +208,15 @@ void modulecmd_arg_free(MODULECMD_ARG *arg);
  */
 bool modulecmd_arg_is_present(const MODULECMD_ARG *arg, int idx);
 
+/**
+ * @brief Check if module command requires an output DCB
+ *
+ * @param cmd Command to check
+ *
+ * @return True if module requires a DCB for printing output
+ */
+bool modulecmd_requires_output_dcb(const MODULECMD* cmd);
+
 /**
  * @brief Call a registered command
  *