Simplify compatibility by simply exposing the POSIX interface as
posix_ptsname_r and use #define to provide an alias when requested.

Amend change to address kib@'s concerns. With this change we now:

In D42470#969304, @kib wrote:

Maybe we should add something like posix_ptsname_r() instead, and #define ptsname_r() to posix_ptsname_r() under the desired POSIX_VERSION. Then we need to add new right POSIX_VERSION, but this sounds better (for me) than randomly breaking ABI.

I'd probably use zstdcat -q here but change LGTM as-is too.

LGTM.

Add regression test for installing empty files.

Address des@'s review: Use putenv() instead of duplicating the work.

Address des@'s comment: Move application of PAM environment to a better place.

I think what bz@ really wanted is something like: