zsh_completion: fix zsh completion installation

Moved zsh shell completion to shell-completion/zsh/_systemd for
automake's sake. Also allow users to specify where the files should go
with::

  ./configure --with-zshcompletiondir=/path/to/some/where

and by default going to `$datadir/zsh/site-functions`
zsh_completion: Split out zsh _systemctl
zsh_completion: Split out zsh _loginctl
zsh_completion: Split out zsh _hostnamectl
zsh_completion: Split out zsh _journalctl

Re-ordered some of the options and added a few that were missing
previously as well.
zsh_completion: Split out zsh _localectl
zsh_completion: Split out zsh _coredumpctl
zsh_completion: Split out zsh _timedatectl
zsh_completion: Split out zsh _udevadm
zsh_completion: Split out zsh _systemd-nspawn

Also fix the random lack of completion
zsh_completion: Split out zsh _systemd-inhibit
zsh_completion: Split out zsh _systemd-analyze
zsh_completion: Split out zsh _machinectl
zsh_completion: Remove unused functions
zsh_completion: Fix journalctl's --boot

Actually displays a list of boot ID's and offsets to the user
zsh_completion: Split out zsh _systemd-tmpfiles

You can choose to have systemd-tmpfiles at configuration time, so only
install the completion for this if configured to do so.
zsh_completion: Speed up noncached perf of _journalctl

Splitting things unnecessarily at newlines causes tab completion to take
an extremely long time. Also add a note saying that caching is not good
for journalctl's completion.
coredumpctl: add more debug output

It can be quite useful when somebody confuses _PID with COREDUMP_PID :).
zsh_completion: add "gdb" verb to coredumpctl
tests: add a program for repetitive opening and closing of the journal

Basically wraps an example provided by George McCollister.
Should help with leaks in the future.