hn/reftable "fixes"

Patch 1, was discussed before and might be solved in a better way as an alternative.

Patch 2 and 3 are "nice to have" for portability and hopefully not controversial but could be dropped, if someone feels strongly against it.

Patch 4 is not something I'd found failing anywhere, but the fact that Microsoft mentions it is only supported as an extension and it needs to be supported by the dynamic linker and I couldn't find anything clear about it in POSIX means, that is probably safer this way.

  [PATCH 1/4] fixup! reftable: add a heap-based priority queue for
  [PATCH 2/4] fixup! reftable: implement stack, a mutable database of
  [PATCH 3/4] config.mak.uname: last release and snapshots of Minix
  [PATCH 4/4] reftable: avoid non portable compile time pointer to