Fix NTP kerninfo offset/estimated error scaling

[PierreF]

This pull requests fix #1300

The scaling was taken from ntp-4.2.6, from ntpdc/ntpdc_ops.c , function kerninfo

Tested with ntp from Ubuntu vivid (1:4.2.6.p5+dfsg-3ubuntu6), value are the correct one with this patch.

Code from ntpdc_ops.c looks like:

  [...]
  struct info_kernel *ik;
  double tscale = 1e-6;
  [...]
#ifdef STA_NANO                                                                 
    if (status & STA_NANO)                                                      
        tscale = 1e-9;                                                          
#endif
    (void)fprintf(fp, "pll offset:           %g s\n",
        (int32)ntohl(ik->offset) * tscale);
    (void)fprintf(fp, "pll frequency:        %s ppm\n",
        fptoa((s_fp)ntohl(ik->freq), 3));
    (void)fprintf(fp, "estimated error:      %g s\n",
        (u_long)ntohl(ik->esterror) * 1e-6);

Note: on my proposed patch tscale is always 1e-6 ... I didn't find where/how STA_NANO could be defined. I've assumed it could only be defined with customized/patched build of ntp.

[octo]

STA_NANO appears to come from the Linux kernel, usable via the Linux specific <sys/timex.h>. I'll add an appropriate comment.

[mfournier]

FYI I cherry-picked this patch as 53438c5. Thanks a lot @PierreF !