Win32: implement variable cleanup more similar to POSIX...
Win32: implement variable cleanup more similar to POSIX...

...and less prone to deadlocks. This restarts until all keys are NULL,
in case cleaning up reset one key back to a non-NULL value.
There is still a slight difference with POSIX still. POSIX will restarts
only after all destructors have been invoked, not after the first one.

jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,krichter722/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.1,shyamalschandra/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,vlc-mirror/vlc,jomanmuk/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,krichter722/vlc,jomanmuk/vlc-2.2,krichter722/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,xkfz007/vlc,krichter722/vlc,jomanmuk/vlc-2.1,shyamalschandra/vlc,vlc-mirror/vlc,shyamalschandra/vlc,vlc-mirror/vlc,xkfz007/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,shyamalschandra/vlc,krichter722/vlc