proto: struct thread *td, struct mknodat_args *uap
parms: td, uap
errors:
  - ENOTDIR
  - ENAMETOOLONG
  - ENOENT
  - EACCES
  - ELOOP
  - EPERM
  - EIO
  - ENOSPC
  - EDQUOT
  - EROFS
  - EEXIST
  - EFAULT
  - EINVAL
  - EBADF
