lib: Add an API to GPG verify a commit given a remote

Conceptually we've been moving towards having our GPG verification
paths be per-remote.  The code internally supports this, but we
didn't expose an API to use it conveniently.

This came up when trying to add a new `gpgkeypath` option, since
right now rpm-ostree manually finds keyrings for the remote, and
hence it wasn't looking at the keypath, and said "Unknown key"
in status.

Adding an API fixes this nicely.

Closes: #576
Approved by: giuseppe

src/libostree/libostree.sym

@@ -364,9 +364,17 @@ global:
  *                         NOTE NOTE NOTE
  */
 
+LIBOSTREE_2016.14 {
+global:
+       ostree_repo_verify_commit_for_remote;
+} LIBOSTREE_2016.8;
+
+/* Section for the stable release *after* this development one; don't
+ * edit this other than to update the last number. */
+
 /* Remove comment when first new symbol is added, replace XX with new stable version.
-LIBOSTREE_2016.XX
+LIBOSTREE_2016.XX {
 global:
 	someostree_symbol_deleteme;
-} LIBOSTREE_2016.8;
+} LIBOSTREE_2016.14;
  * Remove comment when first new symbol is added */