Blender Git Loki

BKE link/append: Add optional blendfile handle to libraries.

This enables calling code to deal with the blendfile handle themselves,
BKE_blendfile_link then just borrows, uses this handle and does not
release it.

Needed e.g. for python's libcontext system to use new
BKE_blendfile_link_append code.

Part of T91414: Unify link/append between WM operators and BPY context
manager API, and cleanup usages of `BKE_library_make_local`.