collectd: interface plugin: revert to Linux defaults from BSD-like operations

collectd: interface plugin: revert to Linux defaults from BSD-like operations

Change the interface plugin to operate with collectd's default way for Linux
instead of trying to use BSD functionality with patches.

Openwrt's collectd Makefile has set "enable-getifaddrs" option since r7980
(in 2007, no explanations given for the reasons to add that config option),
causing the addition of 110-net-device-stats.patch in 2010 to fix things,
and then later (with 5.4.1 upgrade in 2014) 150-fix-interface-af-link
to undo BSD-specific changes in collectd code meant for only BSD systems
(as AF_LINK does not exist in Linux).

Revert to collectd's intended way in Linux by removing the config option
and deleting the two corresponding patches.

The same data fieds should be provided, so there is no visbile change to users.

References:
===========

Collectd changes introducing/explaining the option and later breaking things:
http://git.verplant.org/?p=collectd.git;a=commit;h=a05e34a0d4fdb2c15db8226689268a27f6f7163d
http://git.verplant.org/?p=collectd.git;a=commit;h=14740db4b415655abc58307352e406442f9933a0
http://git.verplant.org/?p=collectd.git;a=commitdiff;h=645dadb3fcc466e8880fda4eb23b21ad433631fc

Openwrt:
intro at r7980: https://dev.openwrt.org/changeset/7980
fix1 due to kernel 2.6.36 at r23467: https://dev.openwrt.org/changeset/23467
fix2 due to collectd 5.4.1: https://github.com/openwrt/packages/pull/298

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>