core: Add runtime ostree_check_version()

[Previously](https://github.com/ostreedev/ostree/pull/728) we added compile-time
checking for versions, but there are use cases for runtime checking as well,
because in a number of API calls we use `GVariant` as an API extension
mechanism.

Closes: #735
Approved by: jlebon

apidoc/ostree-sections.txt

@@ -133,6 +133,7 @@ ostree_validate_structureof_dirtree
 ostree_validate_structureof_dirmeta
 ostree_commit_get_parent
 ostree_commit_get_timestamp
+ostree_check_version
 </SECTION>
 
 <SECTION>