Improve dependencies checks in the build process

- 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

Implement the PING on a server or the gateway itself

fix #2575

send_iq_result can have a "from" with just the domain name

Implement PING to in-room participant

ref #2575

Add utils::revstr

Implement PING, user to user only (XMPP and IRC side, using CTCP PING)

ref #2757

Rename iq_responder_callback_t to irc_… and add the equivalent to wait for iqs

Send a proper error on IRC message 438 (nickname change too fast)

fix #2576

Use generic send_presence_error() instead of almost identical specializations

Add two missing type="error" in some presence errors

Merge branch 'v1'

Conflicts:
	src/network/socket_handler.hpp

Fix IPv6 support (sockaddr size)

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

Send the reason of the connection close to the user

Move some members of TCPSocketHandler into the SocketHandler class

Add a level of inheritance above SocketHandler

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.

Fix a typo

Enable assert() even in release mode, in the test_suite

Use systemd-daemon logging prefix if available

Regularly send a notification for the systemd watchdog

Add missing <string> include in timed_events.hpp

fix #2552