[backend] further tweak parsing of the project config

Fixes bug that caused a dangling Macros section not to be closed.
[backend] link_or_copy: unlink the target before linking

Otherwise the code will truncate the file in the cache
if the link already exists for whatever reason.
[backend] fix dclone call, it cannot clone non-refs
[backend] fix hostlabel constraints merging
- 2.4.5 release notes