ntp: fix accepting requests from configured sources

When using server socket to send client requests (acquisitionport 123)
and currently not waiting for a reply, the socket check will fail for
client requests from the source.

The check needs to be moved to correctly handle the requests as from an
unknown source.
test: fix 111-knownclient

The new NTPv4 loop synchronization check fails as clknetsim doesn't
support IP_PKTINFO yet. Use the noselect option to prevent the server to
synchronize to the client.
tempcomp: allow configuration with list of points

In addition to the quadratic function, allow configuration of the
compensation with a file containing list of (temperature, compensation)
points used for linear interpolation and extrapolation.
doc: update tempcomp description