chrony: add configuration parameters

chrony: add configuration parameters

The existing config sections were anonymous, implying multiple can
coexist. Those are now named so that only one shall exist.

Added:
- smoothtime (in case of large frequency offsets)
- systemclock parameters
- logchange (increase awareness of clock drift in syslog)
- maxsources (for peers; internal default: 4)
- prefer (one server over others)
- interleave (xleave - more accurate transmit timestamps - good to have)

Refactored handle_allow() to handle 'list interface' instead of option.
Then only a single section is required.

Signed-off-by: Paul Donald <redacted>