Extend configuration of NTP sources in UCI:
- Add nts option to enable NTS
- Add disabled option to allow inactive sources

Add nts section to UCI with:
- rtccheck option to disable certificate time checks on systems that
  don't have an RTC to avoid the chicken-and-egg problem (it is less
  secure, but still should be better than no NTS at all)
- systemcerts option to disable system certificates
- trustedcerts option to specify path to trusted certificates

Save NTS keys and cookies by default to avoid unnecessary NTS-KE
sessions when restarted or switching back to an already used NTS source.
Also, save the drift to stabilize the clock after chronyd restart.

Signed-off-by: Miroslav Lichvar <mlichvar0@gmail.com>