tree-wide: Use helpers for unlinkat()

We have `ot_ensure_unlinked_at()` for the "ignore ENOENT" case, and
`glnx_unlinkat()` otherwise. Port all in-tree callers to one or the other as
appropriate.

Just noticed an unprefixed error in the refs case and decided to do a tree-wide
check.

Closes: #1142
Approved by: jlebon

src/libotutil/ot-fs-utils.c

@@ -90,6 +90,7 @@ ot_openat_read_stream (int             dfd,
   return TRUE;
 }
 
+/* Like unlinkat() but ignore ENOENT */
 gboolean
 ot_ensure_unlinked_at (int dfd,
                        const char *path,