Make "getenv" return "nil" in case of failure.
Make "getenv" return "nil" in case of failure.

Returning "void" is a PITA, it prevents doing things such as

  var a = getenv("foobar");
  [...test a...]

	* src/object/system.cc (system_getenv): Return "nil" in case
	of failure, not "void".

aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi