dadd: invalid option -- 0 errors on OpenBSD when days/months/years to keep are set to 1

[morgant]

On OpenBSD, which uses dateutils, we get the following error multiple times if days, months, or years to keep are set to 1 in the config file:

dadd: invalid option -- 0

This is because reldate() is called with days/months/years to keep less one, which would be zero, so reldate is calling dadd with an adjustment of -0d or -0m or -0y, which results in the error.

[morgant]

I think we can probably work around this by detecting a zero adjustment and just using date in that particular case.

[morgant]

Fixed by checking for zero offset in reldate()'s OpenBSD case and just using date instead of dadd as a we don't need to apply an offset anyway.