Deduplicate and fix up our use of mmap()

Buried in this large patch is a logical fix:

```
-  if (!map)
-    return glnx_throw_errno_prefix (error, "mmap");
+  if (map == (void*)-1)
+    return glnx_null_throw_errno_prefix (error, "mmap");
```

Which would have helped me debug another patch I was working
on.  But it turns out that actually correctly checking for
errors from `mmap()` triggers lots of other bugs - basically
because we sometimes handle zero-length variants (in detached
metadata).  When we start actually returning errors due to
this, things break.  (It wasn't a problem in practice before
because most things looked at the zero size, not the data).

Anyways there's a bigger picture issue here - a while ago
we made a fix to only use `mmap()` for reading metadata from disk
only if it was large enough (i.e. `>16k`).  But that didn't
help various other paths in the pull code and others that were
directly doing the `mmap()`.

Fix this by having a proper low level fs helper that does "read all data from
fd+offset into GBytes", which handles the size check. Then the `GVariant` bits
are just a clean layer on top of this. (At the small cost of an additional
allocation)

Side note: I had to remind myself, but the reason we can't just use
`GMappedFile` here is it doesn't support passing an offset into `mmap()`.

Closes: #1251
Approved by: jlebon

src/libostree/ostree-repo-pull.c

@@ -1228,8 +1228,8 @@ meta_fetch_on_complete (GObject           *object,
 
   if (fetch_data->is_detached_meta)
     {
-      if (!ot_util_variant_map_fd (fd, 0, G_VARIANT_TYPE ("a{sv}"),
-                                   FALSE, &metadata, error))
+      if (!ot_variant_read_fd (fd, 0, G_VARIANT_TYPE ("a{sv}"),
+                               FALSE, &metadata, error))
         goto out;
 
       if (!ostree_repo_write_commit_detached_metadata (pull_data->repo, checksum, metadata,
@@ -1245,8 +1245,8 @@ meta_fetch_on_complete (GObject           *object,
     }
   else
     {
-      if (!ot_util_variant_map_fd (fd, 0, ostree_metadata_variant_type (objtype),
-                                   FALSE, &metadata, error))
+      if (!ot_variant_read_fd (fd, 0, ostree_metadata_variant_type (objtype),
+                               FALSE, &metadata, error))
         goto out;
 
       /* Write the commitpartial file now while we're still fetching data */