dns plugin: improve 'data' string encoding/passing

dns plugin: improve 'data' string encoding/passing

encode the full multi-line string as base64 single-line string on
each config write, and decode at config parse time. pass both the data
key/value pairs and the secret txtvalue via STDIN instead of as command
line arguments.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>