smurf: Correct sun number in help text
kappa: Allow an external default to be specified when getting a config value

The new defult is used if no value can be obtained any other way, or if
the obtained value is <undef>.
smurf: Use fixed defaults for skyloop numiter and maptol if no other values are available.
smurf: Add extra diagnostics to makemap

See new "diag.xxx" paremeters in smurf_makemap.def