Make switch statement exhaustive

Now required by lasted stdlib.

dnsc: use constant as it was intended

Signed-off-by: Conrad Hoffmann <ch@bitfehler.net>

Add `dnsc` usage to README

Signed-off-by: Conrad Hoffmann <ch@bitfehler.net>

gitignore: update to current command names

dnsc: support querying DNSSEC record types

While at it, fix dnsx::string(), which I accidentally only converted
halfway from writing to stdout to returning a string...

Signed-off-by: Conrad Hoffmann <ch@bitfehler.net>

**plopp**

Signed-off-by: Conrad Hoffmann <ch@bitfehler.net>