agent: Add option to open socket as specific user

This can prevent an attack where user A passes the path to user B's
ssh-agent socket to the daemon that is running as root.

src/libstrongswan/utils/capabilities.h

@@ -50,6 +50,12 @@ typedef struct capabilities_t capabilities_t;
 #ifndef CAP_SETPCAP
 # define CAP_SETPCAP 8
 #endif
+#ifndef CAP_SETUID
+# define CAP_SETUID 7
+#endif
+#ifndef CAP_SETGID
+# define CAP_SETGID 6
+#endif
 
 /**
  * POSIX capability dropping abstraction layer.