Trivial fixes in core/pkg/net

[anfernee]

Naive PR.

• I feel string comparison is hacky to judge an IP is IPv6 or not. Simplify util method IsIPV6.
• Fixes parsing of /etc/resolv.conf

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[k8s-reviewable]

This change is 

[anfernee]

CLA signed

[coveralls]

Coverage decreased (-0.01%) to 44.427% when pulling 179eee8 on anfernee:master into ed3803c on kubernetes:master.

[aledbf]

@anfernee please squash the commits

[aledbf]

/lgtm

[coveralls]

Coverage decreased (-0.05%) to 44.388% when pulling f2d9d15 on anfernee:master into ed3803c on kubernetes:master.

[anfernee]

@aledbf thanks for reviewing. There should be squash and merge button to merge in github. I don't need to squash it, right?

[aledbf]

@anfernee please squash manually.

[coveralls]

Coverage decreased (-0.03%) to 44.414% when pulling 99227da on anfernee:master into ed3803c on kubernetes:master.

[aledbf]

@anfernee thanks!