This commit adds code for generating keys with dnssec-keygen given
a specific dnssec-policy.
The dnssec-policy can be set with a new option '-k'. The '-l'
option can be used to set a configuration file that contains a
specific dnssec-policy.
Because the dnssec-policy dictates how the keys should look like,
many of the existing dnssec-keygen options cannot be used together
with '-k'.
If the dnssec-policy lists multiple keys, dnssec-keygen has now the
possibility to generate multiple keys at one run.
Add two tests for creating keys with '-k': One with the default
policy, one with multiple keys from the configuration.
