inotify_add_watch flag to prevent modifying existing watch descriptors,
when used on an already watched inode it fails with EEXIST.
added in linux commit 4d97f7d53da7dc830dbf416a3d2a6778d267ae68
this was wrong since the original commit adding inotify, and I don't
see any explanation for it. not even the man pages have it wrong. it
was most likely a copy-and-paste error.