Timers: Parse and format functions for microsecond times

Date/time output (e.g. in logs, show commands) can use %f to specify
subsecond time. By default, millisecond precision is used in output.

nest/password.h

@@ -10,15 +10,13 @@
 #ifndef PASSWORD_H
 #define PASSWORD_H
 
-#include "sysdep/unix/timer.h"
-
 struct password_item {
   node n;
   char *password;			/* Key data, null terminated */
   uint length;				/* Key length, without null */
   uint id;				/* Key ID */
   uint alg;				/* MAC algorithm */
-  bird_clock_t accfrom, accto, genfrom, gento;
+  btime accfrom, accto, genfrom, gento;
 };
 
 extern struct password_item *last_password_item;