Support branchporting without screwing up the dungeon graph

[samfoo]

Using the Eye of Aethiopica you can branchport between dungeon branches. The behavior of branchporting is currently undefined and likely to break... everything. Fix this.

[TjrGithub]

Level teleporting can also switch dungeon branches, but only up. For example, you can level teleport from Vlad's Tower into Vlad's, the Gehennom levels above the top tower level, and all of the Dungeons of Doom.

Perhaps a command to manually update dungeon branches would be a good stopgap measure.

[samfoo]

Level teleporting should currently work correctly even if it switches branches (unless there's a bug). I think you might be right about manually setting the current branch, though, since nothing is perfect. I'll look into making this work tonight.