beginners project: RENAME_EXCHANGE

[Christoph Hellwig]

As people are looking for beginners projects one in a while there's
an easy one at the moment:  implement support for the RENAME_EXCHANGE
flag to the renameat2 system call, which swaps the names for two
existing files.  Documentation here:

http://man7.org/linux/man-pages/man2/rename.2.html

This should mostly be doable by refactoring (or more likely copy &
pasting) the existing rename implementation.  xfstests already has test
case for it, although making sure they covering everything would be a
nice extrea.

(if no one replies to the list I will add it to the wiki after a while)