Rename core.repo-finders to core.default-repo-finders

This renames a config key to make its semantics more obvious. Despite
what the commit message says, it only applies when a set of repo finders
is not specified (either on the command line or in a library API call).
This also renames the corresponding ostree_repo_get function. We can do
this since it hasn't been released yet.

Closes: #1763
Approved by: pwithnall

apidoc/ostree-sections.txt

@@ -298,7 +298,7 @@ ostree_repo_get_mode
 ostree_repo_get_min_free_space_bytes
 ostree_repo_get_config
 ostree_repo_get_dfd
-ostree_repo_get_repo_finders
+ostree_repo_get_default_repo_finders
 ostree_repo_hash
 ostree_repo_equal
 ostree_repo_copy_config