hooks for getenv, putenv, setenv and unsetenv. This makes it
possible for the plugin to trap changes to the environment made by
authentication methods such as PAM or BSD auth so that such changes
are reflected in the environment passed back to sudo for execve().
Set adminstrative domain for the process when looking up user's
password or group info and when preparing for execve().
Include strings.h even if string.h exists since they may define
different things. Fixes warnings on AIX and others.
