Add (and prefer) “prefer” alias to --inode-file-handles

As written in #20 (closed), the current name of the fallback mode for --inode-file-handles can be misleading. This series first adds a prefer alias, then deprecates the fallback value, and finally changes the internal InodeFileHandlesMode enum to call it Prefer instead of Fallback, too.

Note: Patch 3 will conflict with !96 (merged).