server: support readline history / persistence

When using readline, remember the last 50 commands; persist.

lib/mu-runtime.h

@@ -47,6 +47,7 @@ void mu_runtime_uninit (void);
 typedef enum {
 	MU_RUNTIME_PATH_XAPIANDB,   /* mu xapian db path */
 	MU_RUNTIME_PATH_BOOKMARKS,  /* mu bookmarks file path */
+	MU_RUNTIME_PATH_CACHE,      /* mu cache path for attachments etc. */
 	MU_RUNTIME_PATH_MIMECACHE,  /* mu cache path for attachments etc. */
 	MU_RUNTIME_PATH_LOGDIR,     /* mu path for log files */
 
@@ -59,7 +60,7 @@ typedef enum {
  * @return ma string which should be not be modified/freed, or NULL in
  * case of error.
  */
-const char* mu_runtime_path   (MuRuntimePath path);
+const char* mu_runtime_path (MuRuntimePath path);
 
 G_END_DECLS