auth-pam: don't leak PAM env strings after pam_putenv()

[Avinash Duduskar]

The strdup-on-entry contract is stable across all auditable
releases of each implementation:

- Linux-PAM 0-72 (~2001): libpam/pam_env.c pam_putenv ends
  the value-setting path with
    pamh->env->list[item] = _pam_strdup(name_value);
  strdup on entry.

- OpenPAM 20050616 (Jun 2005): lib/pam_putenv.c, both
  replace and append paths call strdup(namevalue).

- illumos 7c478bd (OpenSolaris Launch, 14 Jun 2005):
  usr/src/lib/libpam/pam_framework.c splits name_value
  into local name and value buffers, stores strdup'd
  copies into the env_list. The caller's input is never
  retained.

Pre-tag Linux-PAM (pre-2001) and pre-OpenSolaris-Launch
Sun PAM are outside any plausible current deployment
window.