Introduce TKM specific charon daemon (charon-tkm)

Analogous to charon-nm the charon-tkm daemon is a specialized charon
instance used in combination with the trusted key manager (TKM) written
in Ada.

The charon-tkm is basically a copy of the charon-nm code which will
register it's own TKM specific plugins.

The daemon binary is built using the gprbuild utility. This is needed
because it uses the tkm-rpc Ada library and consequently the Ada
runtime. gprbuild takes care of the complete binding and linker steps
required to properly initialize the Ada runtime.

src/charon-tkm/build_common.gpr

@@ -0,0 +1,10 @@
+with "tkmrpc_client";
+
+project Build_Common is
+
+   for Source_Dirs use ();
+
+   Obj_Dir           := "obj";
+   Compiler_Switches := ("-W", "-Wall", "-Wno-unused-parameter");
+
+end Build_Common;