Avoid doing repetitive calculations, if we call jidprep() on the same JID multiple times

See http://fedoraproject.org/wiki/Packaging:Guidelines#PIE

- Rename all Find*.cmake files to uppercase, to make things more consistent,
  and fix some issues with them (notably the REQUIRED flag)

- Rename SYSTEMDDAEMON to SYSTEMD and only use the libsystemd instead of
  libsystemd-daemon because it's deprecated for a long time now

- Provide a WITH_* and WITHOUT_* switch for all optional dependencies

- Document things in the INSTALL file

fix #2575

ref #2575

ref #2757

Conflicts:
	CMakeLists.txt

fix #2576

Conflicts:
	src/network/socket_handler.hpp

Since struct sockaddr is too small to contain an IPv6, we use struct
sockaddr_in6 instead, and we cast it where needed

Use them to build without linking to them, even if they are on your system

SocketHandler has been renamed to TCPSocketHandler
SocketHandler is now a simple interface with a few methods, used only by
Poller.

This way we can inherite from the new SocketHandler class, to handle other
types of sockets, and still make them manageable by the poller without any
change in the Poller class.