Use more distinctive include guards

Too short include guards are unsafe for the reuse of header files.
This closes #1.
HTTPServer: use only asynchronous-signal-safe functions in the stop() function as it is called by a POSX signal handler.

We therefore replace the pthread_cond_var by an eventFD.
This fixes #5 and closes #4.
Use of angle brackets around file names for include statements

This closes #2.