Allow the rdcd to be started by user other than rdc or root

Remove the check whether the rdcd is started by rdc user.
Add the read access check for the private key and certificates if
the authentication is enabled.

Change-Id: I0e7a7eafb7985801572f809da0cb3e4012683153

include/rdc/rdc.h

@@ -23,6 +23,10 @@ THE SOFTWARE.
 #ifndef INCLUDE_RDC_RDC_H_
 #define INCLUDE_RDC_RDC_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif  // __cplusplus
+
 #include <cstdint>
 
 /** \file rdc_lib.h
@@ -803,4 +807,8 @@ const char* rdc_status_string(rdc_status_t status);
  */
 const char* field_id_string(uint32_t field_id);
 
+#ifdef __cplusplus
+}
+#endif  // __cplusplus
+
 #endif  // INCLUDE_RDC_RDC_H_