win32: better fix for empty environment variables

It appears the CRT and OS each have a copy of the environment.
mingw_putenv() fools the CRT into accepting an empty environment
variable by calling _putenv("V=0") then truncating the new value
by hand.  But _putenv() also updates the OS environment with the
fake 'V=0' value.  Commit 5b48ca53b (win32: pass NULL to spawnve,
not environ) resulted in this fake value being used and hence empty
variables getting the value '0'.

- Add a call to SetEnvironmentVariable() in mingw_putenv() to update
  the OS environment.

- Restore the use of NULL environment pointers in mingw_spawnvp().

- Add a test.

(GitHub issue #250)

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