OPENDNSSEC-620: Extend conf.xml with PidFile and SocketFile

Changes:
* Small changes to enforcer and signer
* Needs extending of the signer client tool, so that it can read conf.xml too
* Needs extending of ods-control, so that it can read the values from conf.xml.
* Needs a new tool: ods-checkconf to extract the value from the conf.xml (used by ods-control)
* New manpage for ods-checkconf