Rename configs that support atypical DNS set ups to put them in hbase.unsafe

HBASE-18226 added a config for disabling reverse DNS checks hbase.regionserver.hostname.disable.master.reversedns and the release note calls out that the config is dangerous:

he following config is added by this JIRA:

hbase.regionserver.hostname.disable.master.reversedns

This config is for experts: don't set its value unless you really know what you are doing.
When set to true, regionserver will use the current node hostname for the servername and HMaster will skip reverse DNS lookup and use the hostname sent by regionserver instead. Note that this config and hbase.regionserver.hostname are mutually exclusive. See https://issues.apache.org/jira/browse/HBASE-18226 for more details.

Caution: please make sure rolling upgrade succeeds before turning on this feature.

We should make clear the risks of using this config by making sure the name starts with hbase.unsafe.

Rename hbase.regionserver.hostname.disable.master.reversedns to hbase.unsafe.regionserver.hostname.disable.master.reversedns but make sure the former is kept with a "deprecated config name" warning.