dns: handle array holes in setServers()

This commit adds better handling of exceptional array formats
passed to dns.setServers(). Prior to this commit, the input
array was validated using map(), which preserves holes, allowing
them to be passed to c-ares, crashing Node. This commit replaces
map() with forEach(), which skips holes.

Fixes: https://github.com/nodejs/node/issues/8538
PR-URL: https://github.com/nodejs/node/pull/8567


Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>