lib: add new extensible memory-type handling

lib: add new extensible memory-type handling

This rewrites Quagga's memory per-type allocation counting, without
using a fixed global list of types.  Instead, source files can declare
memory types which get handled through constructor functions called by
the dynamic linker during startup.

Acked-by: Vincent JARDIN <vincent.jardin@6wind.com>
Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>
[DL: v3: forgot "nonnull" attribute on XFREE]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>