DragonFlyBSD

Just today I needed a version of the "gpt expand" command that doesn't extend the last partition. The attached patch adds a -x option; the last partition is only expanded if -x is specified.

I have done minimal testing:

- I successfully used it without x after copying my laptop's data to a larger SSD, leaving plenty of space at the end for me to install DragonflyBSD on top of the two existing OSs :)

- I did a really quick test with the -x option on a hastily created vnode disk initialized with gpt init. I just verified that after expanding with -x, the Dfly partition got bigger.

I'm happy to do more testing if requested, though I'm not sure exactly what to try.

(This patch changes the default behaviour. I suppose it could be reversed, but a "don't expand the last partition option" seems a bit more convoluted conceptually.)

Patch is vs. commit 42a874b478.