Implement miscfiles_cert_type().
    
    This is based on Fedoras' miscfiles_cert_type implementation.
    The idea was that openvpn needs to be able read home certificates (home_cert_t) which is not implemented in refpolicy yet, as well as generic cert_t certificates.
    
    Note that openvpn is allowed to read all cert_types, as i know that it needs access to both generic cert_t as well as (future) home_cert_t. Dwalsh noted that other domains may need this as well but because i do not know exactly which domains i will not changes any other domains call to generic cert type interfaces.
    
    Signed-off-by: Dominick Grift <domg472@gmail.com>