windows: update to nssm 2.21.1

new 32-bit binary for nssm.exe, upgrading from the previous 2.16.

Has some bug fixes, see http://nssm.cc/changelog.

Driving impetus to upgrade was "Silently ignore INTERROGATE control.",
on win2012 nssm spews ~30 event log messages per minute.

Tested by stopping windows minion, replacing nssm.exe, and restarting.