FreeBSD port utmp plan

[Roland McGrath]

> + Next, decide about the utmp implementation. There are two choices:
> +   utmp-compat    Lets glibc use the same format for utmp and wtmp as FreeBSD
> +                  libc. Recommended if you want to use glibc and FreeBSD libc
> +                  on the same system.
> +                  Configure option: --enable-compatible-utmp
> +   utmp-utmpx     Lets glibc use the extended utmpx format for utmp and wtmp.
> +                  This provides for better Linux source code compatibility.
> +                  Configure option: --disable-compatible-utmp
> + The default is --enable-compatible-utmp.

It's not acceptable to have a configure switch like this that affects the
API/ABI.  The platform is the platform.  The utmp code is already
structured with a dispatch table that makes it reasonably easy to support
multiple file formats, which is the real goal.  Wouldn't it be better to
use the Linux-style formats in the API and just have file-format
compatibility?  Can detect which format the existing file is in, or be
controlled in some other way for what format to write in?