See https://github.com/NixOS/nixpkgs/pull/82131 for the rest of the
changes for FreeBSD. This is PRed separately because it is a macOS
moderate rebuild so we target staging.
The main change is that we CD to the path we're building *after*
applying patches, so we can patch other parts of the tree (from
`extraPaths`) as needed.
Another change is that `netbsd.install` no longer depends on `fts`,
which it evidently no longer needs.
Adds a couple of useful NetBSD and OpenBSD derivations. Some of these
will be integrated into Nixpkgs later.
Noncomprehensive list:
- netbsd.getent
- netbsd.getconf
- netbsd.fts
- openbsd.mg
- netbsd.compat (can replace libbsd)
