sys_netbsd: allow running without root privileges

On NetBSD programs with write access to /dev/clockctl can adjust or set
the system clock without the root privileges. Add a function to drop the
privileges and check if the process has write access to the device to
get a more descriptive error message when the chrony uid/gid doesn't
match the owner of the device.

sys_netbsd.h

@@ -32,4 +32,6 @@ void SYS_NetBSD_Initialise(void);
 
 void SYS_NetBSD_Finalise(void);
 
+void SYS_NetBSD_DropRoot(uid_t uid, gid_t gid);
+
 #endif