net-mgmt/nfdump: upgrade 1.6.17 -> 1.6.18

• add nfpcapd OPTION to generate nfdump netflow records directly from interfaces/pcaps.
• The old COMPAT15 mode has been removed. All old 1.5.x files should have been converted so far.

Relnotes: https://github.com/phaag/nfdump/blob/master/ChangeLog