fix #2533

When the connection is lost, immediately try to reconnect, then try to
reconnect every 2 seconds.  This is much better than the previous “Try to
re-connect as fast as possible”.

fix #2946

Because we need to wait for the welcome message, when we connect to the
server, before sending the JOIN command, we need to also save the value of
the password to reuse it when we actually send the JOIN command

fix #2882

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