Prevent a previously set ErrorLevel env var from preventing result codes

[barries]

Turns out batch files need to clear the ErrorLevel env var, lest a prior batch file's results prevent correct result codes from this invocation.

[Leont]

@haarg @wchristian @dolmen Opinions?

[haarg]

Looks correct. If an ERRORLEVEL environment variable is set, %ERRORLEVEL% will use that rather than giving the exit status. And Win32 has no concept of an existing but empty environment variable, so set VAR= deletes the variable.