Set usrinfo for AIX

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.

plugins/sudoers/parse.c

@@ -31,11 +31,10 @@
 #endif /* STDC_HEADERS */
 #ifdef HAVE_STRING_H
 # include <string.h>
-#else
-# ifdef HAVE_STRINGS_H
-#  include <strings.h>
-# endif
 #endif /* HAVE_STRING_H */
+#ifdef HAVE_STRINGS_H
+# include <strings.h>
+#endif /* HAVE_STRINGS_H */
 #ifdef HAVE_UNISTD_H
 # include <unistd.h>
 #endif /* HAVE_UNISTD_H */