Add basic support for inline-signing.
    
    Inline-signing is enabled for zones with idnsSecInlineSigning attribute = TRUE.
    
    Limitations:
    - Signing configuration is hardcoded in create_zone() as magic constants
    - idnsSecInlineSigning attribute cannot be changed at run-time
    - DNS updates are not supported
    - Signing keys have to be pre-generated and stored in
      <dyndb-ldap working directory>/<ldap intance name>/<zone name>/keys
      directory before named is started
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/56
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>