build: syntax check to avoid 'const fooPtr'

build: syntax check to avoid 'const fooPtr'

'const fooPtr' is the same as 'foo * const' (the pointer won't
change, but it's contents can).  But in general, if an interface
is trying to be const-correct, it should be using 'const foo *'
(the pointer is to data that can't be changed).

Now that the code base has been cleaned, enforce it with a syntax
checker.

* cfg.mk (sc_forbid_const_pointer_typedef): New rule.

Signed-off-by: Eric Blake <eblake@redhat.com>