cmd/syncthing: Clean up deadlock envvars

So STDEADLOCK seems to do the same thing as STDEADLOCKTIMEOUT, except in
the other package. Consolidate?

STDEADLOCKTHRESHOLD is actually called STLOCKTHRESHOLD, correct the help
text.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4598
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