Linux's open returns ENOTDIR when used with O_DIRECTORY|O_NOFOLLOW and
the path is a symlink. Update the code to expect this.
FreeBSD's open returns EMLINK instead of ELOOP when using O_NOFOLLOW on
symlink. Update the code to expect this.
WASI currently lacks the ability to specify the full UNIX access control
information when creating files and directories, so for now just avoid
creating executable files and rely on the umask.
If a path_open call is requesting __WASI_RIGHT_FD_FILESTAT_SET_SIZE,
interpret that as a request for write privleges. If it is requesting
O_TRUNC, require __WASI_RIGHT_PATH_FILESTAT_SET_SIZE, since this is
a path operation rather than a FD operation.
