Initial cut at a hooks implementation. The plugin can register

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().

common/sudo_debug.c

@@ -100,6 +100,7 @@ const char *const sudo_debug_subsystems[] = {
     "rbtree",
     "perms",
     "plugin",
+    "hooks",
     NULL
 };