Move export permission check to LLPermissions, try two.

Removes code duplication.

Several parts of the code ignored the PERM_EXPORT but
and still did demand that things are full perm next
to being a creator. This has now changed the export
rules are the same for everything as they were for mesh:
you need to be the owner and the creator for every element
that is exported (not just the root prim, of course).

indra/llinventory/llpermissions.cpp

@@ -1098,3 +1098,9 @@ LLPermissions ll_permissions_from_sd(const LLSD& sd_perm)
 	rv.fix();
 	return rv;
 }
+
+bool LLPermissions::allowExportBy(LLUUID const& requester, bool supports_export) const
+{
+	return !mIsGroupOwned && requester == mOwner && (requester == mCreator || (supports_export && (mMaskEveryone & PERM_EXPORT)));
+}
+