ifpps: fix matching wrong interface/device

There were a bug in ifpps, that would match the first occurance
of a device/interface name in /proc/net/dev, because it located
the device based on a substring.

If the device name were starting with the same characters,
e.g. having a device named eth4 and a VLAN device named eth4.100.
If specifying --dev eth4, then depending on the order of
/proc/net/dev the wrong device could be choosen for stats. (Note
kernel does not guarantee the order of devices in proc file)

Fix the bug, by matching with a colon ':' appended to the device
name, as the kernel proc format prints a ':' behind the device name.

Fixes #119

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>