Automatically hide the SWAPIN/IO columns when they are unavailable

Automatically hide the SWAPIN/IO columns when they are unavailable

Now that the Linux kernel can enable or disable data collection for them at
runtime, showing the columns when collection is disabled is even less useful,
since the previous data could still be present in the Linux kernel buffers.

Preserve the behaviour of the batch mode though, so that programs parsing
its output aren't broken by the changes to the Linux kernel, but they may
still be broken when the output changes from ?unavailable? to real data.

Since the current sorting keys code makes it hard to dynamically choose which
columns are shown or hidden, when the two columns are hidden, just skip over
displaying them or using them as sorting keys.

Rewrite the data display code to be more flexible wrt column choice though.

Suggested-by: Boian Bonev <bbonev@ipacct.com>