Rename some incomplete gamma functions

Rename the functions to make it clearer what's being computed since
we're using -upper and -lower for the different parts of the
incomplete gamma function.  So:

- `cf-incomplete-gamma-tail` -> `cf-incomplete-gamma-upper`
- `cf-incomplete-gamma` -> `cf-incomplete-gamma-lower`
- `s-incomplete-gamma` -> `s-incomplete-gamma-lower`
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passed test #7527