libpcp_pmda: ensure af_unix socket ownership allows pmcd to read

libpcp_pmda: ensure af_unix socket ownership allows pmcd to read

Now that PMDAs-as-root can be started by pmcd-not-as-root, we can
enter a situation where comms sockets created by PMDAs cannot be
accessed by pmcd.  Fix this by allowing the user pmcd runs as to
have access to these sockets (via filesystem permissions setting
on the socket file).

Problem uncovered by qa/110.