vici: Support completely asynchronous initiating and termination

In some situations the vici client is not interested in waiting for a
timeout at all, so don't register a logging callback if the timeout argument
is negative.

src/libcharon/plugins/vici/README.md

@@ -266,6 +266,9 @@ Initiates an SA while streaming _control-log_ events.
 		errmsg = <error string on failure or timeout>
 	}
 
+The default timeout of 0 waits indefinitely for a result, and a timeout value
+of -1 returns a result immediately.
+
 ### terminate() ###
 
 Terminates an SA while streaming _control-log_ events.
@@ -282,6 +285,9 @@ Terminates an SA while streaming _control-log_ events.
 		errmsg = <error string on failure or timeout>
 	}
 
+The default timeout of 0 waits indefinitely for a result, and a timeout value
+of -1 returns a result immediately.
+
 ### install() ###
 
 Install a trap, drop or bypass policy defined by a CHILD_SA config.