htdecodetoken should use scitokens-verify stdin when available

scitokens-cpp 1.2.0 can now read a token from stdin.  Use that if available.  It can be distinguished by the error message if you try to run it with no parameters and a token in stdin.  The first line from the old version said
```
scitokens-verify: Insufficient arguments; must at least provide a token.
```