putenv: port better to native Windows

putenv: port better to native Windows

* lib/putenv.c [(_WIN32 || __WIN32__) && ! __CYGWIN__]:
Define WIN32_LEAN_AND_MEAN and include <windows.h>.
(_unsetenv): Use _putenv if available.
(putenv): Temporarily set NAME=' ' rather than NAME='x' as that's
a bit less likely to cause damage.
(putenv) [(_WIN32 || __WIN32__) && ! __CYGWIN__]:
Fix the wrong value with SetEnvironmentVariable.
(putenv) [!HAVE__PUTENV]: Simplify and match the HAVE__PUTENV
code better.