Fix issue 10962 - improved handling of gammaIncompleteCompleInverse edge cases

[tedgin]

This resolves issue #10962.

• Made NaN handling conform to that of builtin binary operators, i.e., when both input values are NaN, the one with the larger payload is returned.
• Extended domain coverage to handle a = +0
• Short circuit cases where Q isn't invertible and return NaN
• Explicitly handle p = 1 edge case.

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dub run digger -- build "master + phobos#10963"