9pfs: introduce macOS host support and cleanup

* Add support for Darwin (a.k.a. macOS) hosts.

* Code cleanup (move qemu_dirent_dup() from osdep -> 9p-util).

* API doc cleanup (convert Doxygen -> kerneldoc format).

# gpg: Signature made Mon 07 Mar 2022 11:14:45 GMT
# gpg:                using RSA key 96D8D110CF7AF8084F88590134C2B58765A47395
# gpg:                issuer "qemu_oss@crudebyte.com"
# gpg: Good signature from "Christian Schoenebeck <qemu_oss@crudebyte.com>" [unknown]
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: ECAB 1A45 4014 1413 BA38  4926 30DB 47C3 A012 D5F4
#      Subkey fingerprint: 96D8 D110 CF7A F808 4F88  5901 34C2 B587 65A4 7395

* remotes/cschoenebeck/tags/pull-9p-20220307:
  fsdev/p9array.h: convert Doxygen -> kerneldoc format
  9pfs/coth.h: drop Doxygen format on v9fs_co_run_in_worker()
  9pfs/9p-util.h: convert Doxygen -> kerneldoc format
  9pfs/9p.c: convert Doxygen -> kerneldoc format
  9pfs/codir.c: convert Doxygen -> kerneldoc format
  9pfs/9p.h: convert Doxygen -> kerneldoc format
  9pfs: drop Doxygen format from qemu_dirent_dup() API comment
  9pfs: move qemu_dirent_dup() from osdep -> 9p-util
  9p: darwin: meson: Allow VirtFS on Darwin
  9p: darwin: Adjust assumption on virtio-9p-test
  9p: darwin: Implement compatibility for mknodat
  9p: darwin: Compatibility for f/l*xattr
  9p: darwin: *xattr_nofollow implementations
  9p: darwin: Move XATTR_SIZE_MAX->P9_XATTR_SIZE_MAX
  9p: darwin: Ignore O_{NOATIME, DIRECT}
  9p: darwin: Handle struct dirent differences
  9p: darwin: Handle struct stat(fs) differences
  9p: Rename 9p-util -> 9p-util-linux
  9p: linux: Fix a couple Linux assumptions

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>