conf file XML syntax checker missing supported fields <Signer><PidFile> and <Signer><SocketFile>

The ODS Signer supports <PidFile> and <SocketFile> configuration options but the XML syntax checker does not know of them and does not allow them to be entered. Each Update of the program, I must manually edit the conf.rng file to copy these element specifications from the <Enforcer> section to the <Signer> Section. This change should be made upstream.

In https://github.com/opendnssec/opendnssec/blob/develop/conf/conf.rnc

lines 106 - 110

# File used to detect enforcerd is already running.
& element PidFile { xsd:string }?
# Socket to use for communicating between enforcer and enforcerd
& element SocketFile { xsd:string }?

Need to be duplicated into the "element Signer" section.