refactor error message-related functions

split message print fuctions (mpr_*), strerrno, and mscp_get/set_error
into print.c/h and strerrno.c/h.

ToDo: revise usages of priv_set_errv and pr_* functions.

src/path.h

@@ -10,7 +10,6 @@
 #include <list.h>
 #include <atomic.h>
 #include <ssh.h>
-#include <message.h>
 
 struct path {
 	struct list_head	list;	/* mscp->path_list */