This reverts commit 54f96deb.

Because apparently when cloning a repository with submodules, git creates
empty sub-directories

By using SYSTEMD_FOUND instead of SYSTEMDDAEMON_FOUND, where I forgot to
rename it…

fix #2533

- 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

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

fix #2551

See CMP0037

For now, it tries two TLS ports and then connects to the non-tls port.  In
the future we would like the user to be able to configure that.

fix #2435