Handle the various pkcs12/cert/key/ca combinations.

OpenVPN allows supplying PKCS#12 client cert/key *or* PEM client cert/key, as
well as a PEM CA cert in either case. When supplying --pkcs12, the args --cert
and --key can not be used. So, you can have one of the following:

	- PKCS#12 combined cert/key/ca (one file): --pkcs12
	- PKCS#12 combined cert/key/ca with PEM CA: --pkcs12, --ca
	- PEM cert/key, PEM CA: --cert, --key, --ca

If a user tries to import an invalid config, they will be met with the appropriate
error message. The editor dialog also enforces the PKCS#12/PEM restrictions.
2 jobs for issue-51 in 5 minutes and 47 seconds
Status Job ID Name Coverage
  Build
passed #945182
fedora28_dist

00:03:01

 
  Test
passed #945183
fedora_from_dist

00:02:45