There was an extra DISABLED=no line written to interface configurarion
that's been superceded by the subsequently added parametrized one;
just drop it.

Thanks glebfm@ for spotting the garbage.

It's hardwired at 1/10 of the default /etc/net value
since 3 seconds are enough for properly functioning
DHCP servers in properly maintained networks (those
improper ones tend to have problems with 30 seconds
anyways), and waiting for too long makes users feel
bad for a reason.

Thanks msp@ for bringing attention to this.

It came unnoticed that USE_IFPLUGD clashes with
CONFIG_WIRELESS which is the default for eth ifaces.

Setting NM_CONTROLLED is apparently not enough to disable
/etc/net handling of a particular interface; thanks sem@
for noticing the fortunate error messages in logs
and explaining this peculiarity to me.

This one is also putting an end to an overly long lived hack
named vm-net by replacing it with a bit more generic mechanism.