sys-linux: rework get_first_ethernet()

We can't assume that host has ethernet NIC named "eth0". Rather than guessing we
better ask udev. We iterate over symlinks symlinks in /sys/class/net and
for each device we determine if it is ethernet device and additionally we query
udev database for sub-type of the device. If we find PCI or USB device which has
ethernet datalink type and appropriate sub-type we return its name. If don't
succeed in determining more information about device we will return "good
enough" device which in turn is first device with ethernet datalink type.

Note that we now have two copies of get_first_ethernet() in the source code. This
is bad and should be fixed in the future.

This commit replaces ppp-2.4.5-eth.patch.

Resolves: #682381
pppd: put lock files in /var/lock/ppp

Resolves: #708260
build-sys: compile pppol2tp plugin with RPM_OPT_FLAGS
build-sys: compile pppol2tp with multilink support

Resolves: #817013
build-sys: make pppd first and then plugins
build-sys: install rp-pppoe plugin files with standard perms