[$] Flags for fchmodat()

The fchmodat()system call on Linux hides a little secret: it does not actually implementall of the functionality that the man page claims (and that POSIXcalls for). As a result, C libraries have to do a bit of a complicatedworkaround to provide the API that applications expect. That situationlooks likely to change with the 6.6 kernel, though, as the result of this patchseries posted by Alexey Gladkov.