Modernize "memcached" module

Modernize the memcached module like the other modules, so that:

1. One can get and compile memcached.so by doing "make" in apps/memcached,
   outside of OSv's compilation.

2. Prepare for "make image=memcached" to work (this also needs a patch
   to the main repository to add images/memcached.py).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>