Prepare for removing most of the X509_TRUST API

Prepare for removing most of the X509_TRUST API

X509_check_trust() is of course used by the verifier. Unfortunately
M2Crypto exposes it. The only other part of the X509_TRUST API that
are still needed are the X509_TRUST_* macros in x509.h, as they are
used via *_set_trust and indirectly via the purpose stuff. The rest
will be removed.

X509_TRUST_add() was defanged recently, in particular it no longer
hangs strdup()'ed strings off the global struct. Nothing ever cleaned
these up. TRUST_cleanup() attempted to do so, but since it checked
the dynamic/dynamic strings flags in the wrong order, that cleanup
call ended up doing nothing, so that code was removed at some point.

As a consequence, the struct can now be made const. Use a CTASSERT()
to ensure size assumptions on X509_TRUST_COUNT, X509_TRUST_MAX, and
X509_TRUST_MIN hold true.

Remove the global variable underlying X509_TRUST_set_default()'s
functionality and move its accessor down to all the other functions
that will be deleted.

Inline a few things in X509_check_trust(), so we can excise the
internals of X509_TRUST_get0(), X509_TRUST_get_by_id(). Since the
default trust function can no longer be changed, call obj_trust()
directly.

ok jsing