libsmartcols: (filter) Add on-demand data filler

libsmartcols: (filter) Add on-demand data filler

When dealing with really large data sets, we need to reduce overhead
by filtering out unnecessary lines. The traditional approach, using:

 lstool | grep <expr>

requires the tool to read all data from the system and then filter out
(and throw away) a lot of data.

The filter-filler now allows us to use an empty line. The filter will
request data through a callback, and when a line passes the filter,
the application can fill in the rest of the columns.

For example, in a query like "FOO > 10 && BAR < 10," libsmartcols will
never ask for "BAR" if "FOO" is smaller than 10. This means the
application doesn't have to gather additional columns.

Signed-off-by: Karel Zak <kzak@redhat.com>