Zero args runner "allzero" behavior

[Erotemic]

Zero args runner should ignore zero-args funcs that have doctests.

In ubelt python -m ubelt zero runs _win32_can_symlink, which has a doctest probably shouldn't be run by the xdoctest "zero" command. At least there should be a way to disable it from running.

Running this should still work

python -m xdoctest /home/joncrall/code/ubelt/ubelt/_win32_links.py _win32_can_symlink

But running

python -m ubelt zero

should skip any zero-args function that has a doctest by default.