Add metrics from SNTPv4 packet to ntp collector & add ntpd sanity check (#655)

* 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

1 files changed, 1 insertions, 1 deletions

diff --git a/README.md b/README.md
index 3565bcc..379210f 100644
--- a/README.md
+++ b/README.md
@@ -64,6 +64,7 @@ logind | Exposes session counts from [logind](http://www.freedesktop.org/wiki/So
 meminfo\_numa | Exposes memory statistics from `/proc/meminfo_numa`. | Linux
 mountstats | Exposes filesystem statistics from `/proc/self/mountstats`. Exposes detailed NFS client statistics. | Linux
 nfs | Exposes NFS client statistics from `/proc/net/rpc/nfs`. This is the same information as `nfsstat -c`. | Linux
+ntp | Exposes local NTP daemon health to check [time](./docs/TIME.md) | _any_
 qdisc | Exposes [queuing discipline](https://en.wikipedia.org/wiki/Network_scheduler#Linux_kernel) statistics | Linux
 runit | Exposes service status from [runit](http://smarden.org/runit/). | _any_
 supervisord | Exposes service status from [supervisord](http://supervisord.org/). | _any_
@@ -78,7 +79,6 @@ Name     | Description | OS
 ---------|-------------|----
 gmond | Exposes statistics from Ganglia. | _any_
 megacli | Exposes RAID statistics from MegaCLI. | Linux
-ntp | Exposes time drift from an NTP server. | _any_
 
 ### Textfile Collector