kern_linkat: modify to accept AT_ flags instead of FOLLOW/NOFOLLOW

[trasz]

Summary

This makes this API match other kern_xxxat() functions.

[trasz]

Not that it's not tested yet; advice on how to exercise this functionality is welcome.

[kib]

You should write (or find existing) test that uses linkat(AT_SYMLINK_FOLLOW) and linkat(0) on the symlink. In first case the link to the target should be created, in the second case perhaps error is returned (not sure which).

[trasz]

(Tinderboxed.)

[trasz]

Okay, it appears to work; I've tested with linkat_test, and also this:

#include <assert.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

int
main(int argc, char **argv)
{
int error;

error = linkat(AT_FDCWD, "name1", AT_FDCWD, "name2-bez-linkat", 0);
perror("linkat");

error = linkat(AT_FDCWD, "name1", AT_FDCWD, "name2-z-linkat", AT_SYMLINK_FOLLOW);
perror("linkat");


return (0);
}

Result, in both cases (with, and without the patch), is:

lrwx------  2 trasz  trasz    11 Apr 25 14:08 name2-bez-linkat -> /etc/passwd
-rw-r--r--  2 root   wheel  3801 Mar 30 14:42 name2-z-linkat