Fix some warnings given by Debian's lintian tool

When building Debian package of your empcd project on a Wheezy system (Voyage MPD 0.9.2), I've done some small modifications following lintian tips :

- use '9' compat level to use hardening flags when compiling C sources
- update Makefile to use hardening flags
- remove 'exec' flag on debian/docs, dh tries to execute it !
- fix some compilation warnings in empcd.c
- add a minimal copyright file.