install: create_symlink() check unlink() return value

create_symlink() do not check the return value of unlink(), this may
confuse the user.

Before the unlink() call we check the 'force' argument. If it is not set
we fail with -EEXIST, otherwise we unlink() the file, therefore the next
symlink() should not fail with -EEXIST (do not count races...).

However since callers may not have appropriate privileges to unlink()
the file we lose the -EPERM or any other errno code of unlink(), and
return the -EEXIST of the next symlink(). Fix this by checking unlink()
results.

Before:
$ systemctl --force --root=~/container-03 set-default multi-user.target
Failed to set default target: File exists

After:
$ systemctl --force --root=~/container-03 set-default multi-user.target
Failed to set default target: Permission denied

src/shared/install.c

@@ -1170,7 +1170,9 @@ static int create_symlink(
         if (!force)
                 return -EEXIST;
 
-        unlink(new_path);
+        r = unlink(new_path);
+        if (r < 0 && errno != ENOENT)
+                return -errno;
 
         if (symlink(old_path, new_path) >= 0) {
                 add_file_change(changes, n_changes, UNIT_FILE_UNLINK, new_path, NULL);