libutil/archive: add preallocate-contents option
libutil/archive: add preallocate-contents option

Make archive preallocation (fallocate) optional because some filesystems
like btrfs do not behave as expected with fallocate.

See #3550.

NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix