factor: remove unreachable SQUFOF code at compile time
It was a little confusing as to whether the SQUFOF algorithm was
enabled, and in fact there were no options available to enable it.
Therefore clarify the 3 configurable behaviors for the code to
3 defines at the top of the program, and only include the SQUFOF
code if enabled at compile time.
$ size src/factor-before
text data bss
93997 1412 2504
$ size src/factor-after
text data bss
87885 1404 2504
* src/factor.c: Only include the SQUFOF factor code
when enabled via the USE_SQUFOF define.
* doc/coreutils.texi (factor invocation): Update note about
factor limits, as we can factor 128 bit numbers without GMP.