Run as nonroot user on Linux (with CAP_NET_ADMIN and CAP_NET_RAW added).

- ZT will only drop root privileges if zerotier-one user exists. It is created by
Debian postinst script - in other cases the user has to be created by administrator.
- Linux >=4.3 with ambient capabilities is required, otherwise ZT will silently
- "-U" option now also disables privileges dropping

osdep/LinuxDropPrivileges.hpp

@@ -0,0 +1,9 @@
+#ifndef ZT_LINUXDROPPRIVILEGES_HPP
+#define ZT_LINUXDROPPRIVILEGES_HPP
+#include <string>
+
+namespace ZeroTier {
+    void dropPrivileges(std::string homeDir);
+}
+
+#endif