factor: remove --bignum and --no-bignum options

factor: remove --bignum and --no-bignum options

Here's a patch to remove the --bignum and --no-bignum options from
'factor'.  The case for removing --bignum isn't as strong as that for
'expr', but still, it seems to me that these options are not needed and
complicate the documentation unnecessarily.

* doc/coreutils.texi (factor invocation): Remove --bignum, --no-bignum.
* src/factor.c (algorithm, ALGORITHM_CHOICE, USE_BIGNUM, NO_USE_BIGNUM):
Remove; all uses removed.
(extract_factors_multi): Remove, replacing with....
(print_factors_multi): New function, with signature similar to that
of new signature of print_factors_single.
(print_factors_single): Migrate checking code to caller.
(print_factors): Use GMP if it's available; don't bother asking user.
Improve accuracy of check for "large" numbers.
(long_options, main): Remove support for --bignum.