Use libunbound to manage nametable

[brendonj]

It seems unnecessary to keep a separate list of names to addresses when libunbound can already do that using ub_ctx_hosts(), especially when the current implementation ignores /etc/hosts and uses a similar but incompatible format. Less code to maintain and more consistency with the host environment are good things.

Replace the nametable with libunbound. Load /etc/hosts and any nametable files from the usual locations.

Downsides:

• Can't be reloaded or cleared once the context has been used to perform name resolution. Was that ever used?
• New format isn't backwards incompatible. Will need to update ansible templates and maintain the deprecated nametable format for some time.