libglnx porting: Use glnx_set_error_from_errno

⚠️ There is a notable spiked pit trap here around
`posix_fallocate()` and `errno`.  This has bit other projects,
see e.g.
https://github.com/systemd/systemd/commit/7bb87460e691d30c1a7fd23a1a8240776957e05f

Otherwise the port was straightforward.

src/libostree/ostree-sepolicy.c

@@ -347,10 +347,9 @@ ostree_sepolicy_get_label (OstreeSePolicy    *self,
       res = selabel_lookup_raw (self->selinux_hnd, &con, relpath, unix_mode);
       if (res != 0)
         {
-          int errsv = errno;
-          if (errsv != ENOENT)
+          if (errno != ENOENT)
             {
-              gs_set_error_from_errno (error, errsv);
+              glnx_set_error_from_errno (error);
               goto out;
             }
         }
@@ -443,7 +442,7 @@ ostree_sepolicy_restorecon (OstreeSePolicy    *self,
           int res = lsetfilecon (gs_file_get_path_cached (target), label);
           if (res != 0)
             {
-              gs_set_error_from_errno (error, errno);
+              glnx_set_error_from_errno (error);
               goto out;
             }
         }
@@ -491,7 +490,7 @@ ostree_sepolicy_setfscreatecon (OstreeSePolicy   *self,
 
   if (setfscreatecon_raw (label) != 0)
     {
-      gs_set_error_from_errno (error, errno);
+      glnx_set_error_from_errno (error);
       return FALSE;
     }