Severity: wishlist

Consider doing this in emacs -Q

    M-: (setq sentence-end-double-space nil) RET

Now put point at a paragraph like this

    Foo bar baz.  Hello hello.

Press M-q and end up with:

    Foo bar baz. Hello hello.

However, in some cases the user will _not_ want this be reflowed, while
still wanting to make the sentence commands navigate correctly even if
there is only one space.

For that purpose, I suggest adding a new user option
`fill-sentence-end-double-space', which controls _only_ the behavior
when refilling.  That way you can customize sentence commands separately
from fill commands.

See the attached patch (still lacking NEWS and documentation changes).

Thoughts?