Use shellcheck to rework the code. Use "export" to return variables from
a function call. Further, fix typos.

Signed-off-by: Nick Hainke <vincent@systemli.org>

This tool can be used to automatically create wireguard tunnels. Using
rpcd a new wireguard interface is created on the server where the client
can connect to.

Wiregurad server automatically installs a user and associated ACL to use
the wireguard-installer-server features. The user is called wginstaller
and so is the password.

Get Usage:
  wg-client-installer get_usage --ip 127.0.0.1 --user wginstaller
	--password wginstaller

Register Interface:
  wg-client-installer register --ip 127.0.0.1 --user wginstaller
         --password wginstaller --bandwidth 10 --mtu 1400

Signed-off-by: Nick Hainke <vincent@systemli.org>