rc.inet1: Check for iface in /sys/class/net, not /proc/net/dev

There is a slight problem with things like
  grep lo: /proc/net/dev
especially with tun/tap devices with can have arbitrary names.

Consider a scenario where you name your tap iface "trololo":
$ grep -c lo: /proc/net/dev
2
$

To fix this (quite hypothetical, but still) problem, instead
of simple grepping in /proc/net/dev, check of sysfs path
existence (/sys/class/net/<interface>).

Same fix in if_up() and if_down() for non-loopback interfaces.

While there, also strip colon-whatever from interface name
(what I think was the original idea behind a construct like
grep $(echo ${1}: | cut -f 1 -d :): /proc/net/dev) by using
parameter expansion: ${1%%:*} will remove the longest suffix
matching ":*", ie. colon-and-everything-after.
This is supported by POSIX:
http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html

Alternative solution would be to change the grep pattern to
  grep '^ *lo:' /proc/net/dev

Signed-off-by: Robby Workman <rworkman@slackware.com>

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