The function nmc_print() receives a list of "targets". These are essentially
the rows that should be printed (while the "fields" list represents the columns).

When filling the cells with values, it calles repeatedly get_fcn() on the
column descriptors (fields), by passing each row (target).

The caller must be well aware that the fields and targets are
compatible. For example, in some cases the targets are NMDevice
instances and the target type must correspond to what get_fcn()
expects.

Add another user-data pointer that is passed on along with the
targets. That is useful, if we have a list of targets/rows, but
pass in additional data that applies to all rows alike.

It is still unused.