The system drivers may implement their own slewing which the generic
driver can use to slew faster than the maximum frequency the driver is
allowed to set directly.
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).
