daemon: Change ln, ln-f (hard-link) APIs to use link(2) instead of external ln (RHBZ#895905).

The reasons to do this are twofold:

(a) It's probably a tiny bit faster.

(b) It lets us capture the real errno if the link(2) syscall fails.

The errno is also passed through guestmount, fixing RHBZ#895905:

+ guestmount -a test1.img -m /dev/sda1:/ -m /dev/sda2:/boot /tmp/mnt
+ touch /tmp/mnt/foo
+ cd /tmp/mnt
+ ln foo boot/foo
ln: failed to create hard link ‘boot/foo’ => ‘foo’: Invalid cross-device link
fuse: If guestfs_last_errno returns 0, don't return no error to FUSE layer.

guestfs_last_errno (g) == 0 doesn't mean "no error".  It means the
errno was not captured.  In this case we have to substitute some sort
of errno, so choose EINVAL arbitrarily.