Move the detection of UUID header files to be included portably. This will have no functional effect until MacOS detection is put in place.

suggested by millert ok thomas_adam

queue.h is included portably, so it shouldn't be included directly.

Move a small amount of code from worktree.c to a new file worktree_open.c, which contains everything required to open and close a work tree and inspect some of its basic parameters. This can be used by gotadmin. ok tracey