Add multi-user support

Let users configure clicraft separately from the system using
$XDG_CONFIG_DIR/clicraft or ~/.config/clicraft.
Document multi-user support
Update bash completion and default configs

Make these work with per-user configs.
Fix example crontab

Replace ${exec_prefix} in scripts/cron with its literal value.
Remove extra backslash

The backslash here doesn't act as an escape character, and is
interpreted literally, so it needs to be removed.
Update install doc

* rsync is required for `clicraft sync`
* GitHub downloads no longer work
Update Contributor's Guide

Update the example msg() function to match the one defined in
src/lib/functions.sh .
Update clicraft(1) man page

CMD_TIMEOUT is what matters to `clicraft cmd`, not TIMEOUT.

Explain where SERVER_LOG is documented.

Remove `clicraft sh`, which is not enabled by default and is
documented in clicraft-examples(1).
Fix CMD_LSTRIP

If this option is set to an empty string, don't override it with
the default.
Update clicraft-examples(1) man page

* Clarify that SERVER_VERSION="release"|"snapshot" is replaced
  with the corresponding Minecraft version
* Fix clicraft(1) link
* Minor formatting fixes
Update clicraft-actions(5) man page

* Fix a link to clicraft(1) man page
* Escape a closing bracket so it's not interpreted as markup
* Minor grammar fixes
Change where lock files are stored

Use $TMPDIR/clicraft.$EUID instead of $TMPDIR/clicraft.$USER .
Document $BINDIR