lib/core: Add ostree_validate_remote_name() for remote names

There are a few places in the code where ad-hoc validation was being
performed. Might as well formalise it a bit more.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #948
Approved by: cgwalters

src/libostree/libostree-devel.sym

@@ -19,7 +19,9 @@
 
 /* Add new symbols here.  Release commits should copy this section into -released.sym. */
 LIBOSTREE_2017.8 {
-};
+global:
+  ostree_validate_remote_name;
+} LIBOSTREE_2017.7;
 
 /* Stub section for the stable release *after* this development one; don't
  * edit this other than to update the last number.  This is just a copy/paste