Added .gitattributes
smcp-timer: Added `period_between_timevals_in_cms()`.
coap: Added block option decoding.
smcp-plugtest-client: Improved logging.
configure.ac: Poach the version from `git describe`, if available.
plugtest-server: Fix possible double-free on async requests.
Added support for `IPV6_PKTINFO` in BSD sockets.

This allows us to know the destination address for inbound packets.