MINOR: config: new global directive server-state-base

This new global directive can be used to provide a base directory where
all the server state files could be loaded.
If a server state file name starts with a slash '/', then this directive
must not be applied.

include/types/global.h

@@ -129,6 +129,7 @@ struct global {
 	char *log_tag;                  /* name for syslog */
 	struct list logsrvs;
 	char *log_send_hostname;   /* set hostname in syslog header */
+	char *server_state_base;   /* path to a directory where server state files can be found */
 	struct {
 		int maxpollevents; /* max number of poll events at once */
 		int maxaccept;     /* max number of consecutive accept() */