GitLab

also, do not use server_main.c, compile both .o instead

* make server aware of single client mode with command line flag
* server will quit when last client disconnects
* unix variant also deletes socket
* move why3server binary to libdir

nonblocking

* previous implementations of call_on_file and call_on_buffer are
  removed, and now directly use the prove_file_server function
* now, both blocking and nonblocking reading is supported on the socket
* wait_on_call and query_call are implemented by blocking and
  nonblocking read, together with a result buffer, if results for other
  prover runs are returned
* logging of the server is now optional

Conflicts:
	examples/avl/priority_queue/why3session.xml
	examples/avl/priority_queue/why3shapes.gz

Previously, the code was written in C, but in fact it was not necessary,
the small difference between Windows and Unix could be achieved in
OCaml, too.