sys: add generic timex driver

This is based on sys_linux.c and wrap_adjtimex.c. It's intended for all
systems that support the adjtimex() or ntp_adjtime() system call. The
driver functions can be replaced with extended system-specific versions
(e.g. to control the frequency with the tick field on Linux).

logging.h

@@ -99,6 +99,7 @@ typedef enum {
   LOGF_SysNetBSD,
   LOGF_SysSolaris,
   LOGF_SysSunOS,
+  LOGF_SysTimex,
   LOGF_SysWinnt,
   LOGF_TempComp,
   LOGF_RtcLinux,