factor: add -h/--exponents option

[tdulcet]

GNU coreutils 9.2 added a new -h/--exponents option to the factor command (source):

  factor now accepts the --exponents (-h) option to print factors
  in the form p^e, rather than repeating the prime p, e times.

The change was made in this commit: coreutils/coreutils@f45ab30. (For an additional reference implementation, I also added support for this to my Numbers Tool, which wraps the factor command: tdulcet/Numbers-Tool@1b897cf.)

Example with GNU factor:

$ ./factor -h 3000
3000: 2^3 3 5^3

This is not supported by uutils' factor. Fixing this along with #1559 should make all the GNU tests for factor pass.