Reencryption code cleanup.

- Remove all 'LUKS2_' name prefixes from internal routines
- Make all internal routines prefixed with 'reencrypt_' instead
- Drop few static routines by refactoring
- Rename all variables and routines containing 'pre' prefix to
  contain 'hot' prefix instead (when referring to segments
  undergoing reencryption)
- Rename all variables and routines containing 'after' prefix to
  contain 'post' prefix instead
- Rename all routines prefixed with '_' to 'reencrypt_' instead

lib/luks2/luks2.h

@@ -169,8 +169,8 @@ struct luks2_reenc_context {
 	int reenc_keyslot;
 
 	/* already running reencryption */
-	json_object *jobj_segs_pre;
-	json_object *jobj_segs_after;
+	json_object *jobj_segs_hot;
+	json_object *jobj_segs_post;
 
 	/* backup segments */
 	json_object *jobj_segment_new;