apps: when the 'compat' nameopt has been set, leave it be

XN_FLAG_COMPAT has a unique property, its zero for value.  This means
it needs special treatment; if it has been set (which can only be
determined indirectly) and set alone (*), no other flags should be
set.

(*) if any other nameopt flag has been set by the user, compatibility
mode is blown away.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6382)