Fixed credentials::read() to properly stat the credentials file.

Review: https://reviews.apache.org/r/21445
Added API for managing links.

Review: https://reviews.apache.org/r/20292
Allowed to get/set MTU for a link.

Review: https://reviews.apache.org/r/21288
Added runtime check to the Linux routing library.

Review: https://reviews.apache.org/r/21269
Added internal filter and queueing APIs.

Review: https://reviews.apache.org/r/20781
Added API for managing ICMP packet filters.

Review: https://reviews.apache.org/r/20295
Added API for managing ARP packet filters.

Review: https://reviews.apache.org/r/20296
Added API for managing IP packet filters.

Review: https://reviews.apache.org/r/20297
Added API for getting the default gateway.

Review: https://reviews.apache.org/r/20347
Allowed waiting for a link to be removed.

Review: https://reviews.apache.org/r/21141