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.
skip it. This prevents packages that incorrectly install a file
into /etc/sudoers.d from breaking sudo so easily. Syntax errors
in #includedir files still result in a parse error (for now).
