pop3d: support fcntl locks on NetBSD and OpenBSD

MboxLock already supported it since it locked the whole file,
but POP3D requires more fine-grained locking at file offsets.

I wonder if "struct flock" is old enough for it to be the same
across all the BSDs, it certainly seems so.

I originally considered using C11 `_Generic' support for the
struct offset/type dumping as I have in other projects, but
I am not ready to depend on C11 for this project, yet.

While we're modifying devel/sysdefs-list, add some Linux-only
structs to verify our `pack' templates are correct and remain
so when we encounter new architectures.

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