Correct field size for Netflow v9 field 48.

Despite spec setting the size as a single byte, testing on Cisco 7200
shows the size is set to 4 bytes. Allow source device to override the
default size and make the default size 4 instead of 1.

Discovered from http://www.rfc-editor.org/errata_search.php?rfc=3954
Improve checking of the template and record length.
Move Netflow fields to use "netflow." namespace.