[perl #82098] Fix RT 61543 and remaining part of RT 63237

[perl #82098] Fix RT 61543 and remaining part of RT 63237

Extend bmodpow() to handle negative numbers.

- dist/Math-BigInt/lib/Math/BigInt.pm: Fix bmodpow() code and make the
  documentation style more like that of other methods.

- dist/Math-BigInt/t/bigintpm.inc: Edit test results so they match new
  behaviour, i.e., where earlier a NaN was returned, there are now
  some cases where an integer is returned, since bmodpow() now also
  handles negative numbers. Also include test cases from RT 63237. The
  tests themselves have all been verified to be correct using other
  software.

- dist/Math-BigRat/t/bigratpm.inc: Fix test case so it matches the new
  behaviour of Math::BigInt->bmodinv(). Math::BigRat->bmodinv() only
  handles integers, and is essentially just a front-end to
  Math::BigInt->bmodinv().

- dist/Math-BigInt/t/bare_mbi.t: Increment test count.

- dist/Math-BigInt/t/bigintpm.t: Increment test count.

- dist/Math-BigInt/t/sub_mbi.t: Increment test count.