Prepare move potfile specific functions into their own source file

include/data.h

@@ -178,6 +178,8 @@ typedef struct
 
   hashconfig_t *hashconfig;
 
+  potfile_ctx_t *potfile_ctx;
+
   #if defined (HAVE_HWMON)
   uint    gpu_temp_disable;
   uint    gpu_temp_abort;
@@ -189,8 +191,6 @@ typedef struct
   uint    rp_gen;
   uint    rp_gen_seed;
 
-  FILE   *pot_fp;
-
   /**
    * used for restore
    */