Simplify the library initialization + deprecate 2 XBT functions

Simplify the library initialization + deprecate 2 XBT functions

Work toward making EngineImpl::initialize() the only entry point of
the library initialization. It's now impossible to initialize xbt
separately of EngineImpl.

It should help reducing the mess in that code, which results from the
many ways of initializing the library. simgrid-mc, unit-tests, smpi
and others each have their own way of initializing everything,
resulting in a spagetthi and fragile code. It's a bit of a waste to
initialize an EngineImpl even when you don't want to actually run a
simulation, but easier code is always better.

This commit also includes another one because I git amended locally by
error, sorry for the mess. The other commit was about the deprecation
of xbt_procname and xbt_getpid that were xbt functions relying on S4U.