box: separate access check and function call in box_process_call

box_process_call() uses func_call(), which not only calls the given
function, but also checks that the current user has the right to execute
it. As a result, we can't add auditing for only those function calls
that passed the access check (apparently, there's no reason to log
function calls that failed with an 'access denied' error - we have a
separate audit event for this).

To fix this, let's introduce func_call_no_access_check() helper, which
calls a function without checking access rights, and use it along with
existing func_access_check() in box_process_call(). func_call() is now
an inline function that calls func_access_check() and then on success
func_call_no_access_check().

It's probably wrong that func_call() checks access rights, because this
means that to use a space with a functional index/constraint, the user
needs not only read/write access to the space itself, but also execute
access to the function. I think we should check the right to execute
such function only once - on functional index/constraint creation, not
on every call, but I'm not going to change this now, because nobody's
complained so far, and a change like this needs a proper discussion
anyway.

NO_TEST=refactoring
NO_DOC=refactoring
NO_CHANGELOG=refactoring