Add missing input validation

[mattias-p]

Here are a bunch of commands with invalid arguments that should be caught in the input validation layer.

zonemaster-cli --ds=1            example.  # missing --ds fields
zonemaster-cli --ds=foobar,1,1,1 example.  # invalid keytag
zonemaster-cli --ds=1,foobar,1,1 example.  # invalid algorithm
zonemaster-cli --ds=1,1,foobar,1 example.  # invalid type
zonemaster-cli --ds=1,1,1,foobar example.  # invalid digest
zonemaster-cli --encoding=foobar example.  # invalid encoding
zonemaster-cli --locale=foobar   example.  # invalid locale
zonemaster-cli --hints=/         example.  # file cannot be opened for reading
zonemaster-cli --restore=/       example.  # file cannot be opened for reading
zonemaster-cli --save=/          example.  # file cannot be opened for writing