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_tests.gpr

@@ -0,0 +1,14 @@
+with "build_common";
+
+project Build_Tests is
+
+   for Languages use ("C");
+   for Source_Dirs use ("src/tkm", "tests");
+   for Main use ("test_runner");
+   for Object_Dir use Build_Common.Obj_Dir;
+
+   package Compiler is
+      for Default_Switches ("c") use Build_Common.Compiler_Switches;
+   end Compiler;
+
+end Build_Tests;