Add statx

[hasufell]

TODO:

• [x] decide whether to provide STATX_MNT_ID
• [ ] maybe provide StatxMask defaults for STATX_BASIC_STATS and STATX_ALL
• [x] fix the autoconf thing and use proper if guards (and use FP_CHECK_CONSTS to check for the constants)
• [x] support the same queries as FileStatus
• [x] add support for String and PosixPath

[hasufell]

ghci> import System.Posix.Files.ByteString
ghci> :set -XOverloadedStrings
ghci> exStatus <- getExtendedFileStatus Nothing "/home/hasufell/git/unix/unix.cabal" defaultStatxFlags defaultStatxMask
ghci> fileSizeX exStatus
9318

Seems to somewhat work.