* Add metrics from SNTPv4 packet to ntp collector & add ntpd sanity check
1. Checking local clock against remote NTP daemon is bad idea, local
ntpd acting as a client should do it better and avoid excessive load on
remote NTP server so the collector is refactored to query local NTP
server.
2. Checking local clock against remote one does not check local ntpd
itself. Local ntpd may be down or out of sync due to network issues, but
clock will be OK.
3. Checking NTP server using sanity of it's response is tricky and
depends on ntpd implementation, that's why common `node_ntp_sanity`
variable is exported.
* `govendor add golang.org/x/net/ipv4`, it is dependency of github.com/beevik/ntp
* Update github.com/beevik/ntp to include boring SNTP fix
* Use variable name from RFC5905
* ntp: move code to make export of raw metrics more explicit
* Move NTP math to `github.com/beevik/ntp`
* Make `golint` happy
* Add some brief docs explaining `ntp` #655 and `timex` #664 modules
* ntp: drop XXX comment that got its decision
* ntp: add `_seconds` suffix to relevant metrics
* Better `node_ntp_leap` comment
* s/node_ntp_reftime/node_ntp_reference_timestamp_seconds/ as requested by @discordianfish
* Extract subsystem name to const as suggested by @SuperQ
