apps: added osv-memcached module

osv-memcached application connects to the FreeBSD PF (packet filtering)
interface and handles inbound memcache UDP requests.

For more info look at https://github.com/cloudius-systems/osv/wiki/osv-memcached

New in V2:
   - Changed the target name: osv-memcached -> osv-memcached.so
   - Makefile: inherit the application directory name from the git URL.
   - Makefile: Use $(notdir) instead of $(shell basename).

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