Based on #13, we need more error checking when opening files. The CA and
CSR files are now checked and return proper error messages in case
something goes wrong.
This commit is a complete rebuild of pkictl. Before everything was all
over the place and adding new commands was kind of a hassle.
Now each command has its own file and can be adjusted on a command
basis. Options are still used by the same name, but can now use
different descriptions.