Calling the thread.getId() function through JNI seems to cause JVM
crashes when runtime debugging is enabled, so we avoid it for now.

This allows re-enabling the unused-parameter warnings globally.

The JVM's modified UTF-8 encoding ensures that C-style strings are
properly terminated by a single NUL character.

This exposes buggy header files more eagerly

This way it works with features (such as extra structure fields)
that depend on DEBUG being defined.

The pthreads functions do not set errno. Rather, they return a value
that can be interpreted as errno, but that would need an extra
function.

This is now also reflected in the main build file.

The make files of both agents were unified and the generated
protobuf-c implementation of the protocol has been put to
separate directory.

This comprises several changes:
- send_client_message now uses a reference to ClientMessage
- neither session_start nor session_end gets to use the config
  structure directly, they just get the bits they need
- there is a new method session_send_instrumentation, which
  loads the jars and sends them to the server, instead of this
  being handled in session_start
- the jars are loaded directly into the common buffer