Since the menhir version we use requires ocaml>4.02 we can also
upgrade the required ocaml version to >4.02 and remove the
deprecate String functions.
Also we now activate all warnings except for 4,9 und 27 for regular
code plus a bunch of warnings for the generated code. 4 and 9 are
not really usefull and 27 is deactivated since until the usage
string is printed in a way that requires no newline.
Bug 18394.