This stops unauthenticated users from sending unauthorized emails.

Example:

`swaks -t matteo@mail.saloniamatteo.top --header "Subject: Unauthorized Email" --body "This email should not go through." --server mail.saloniamatteo.top`

Before this patch, any user could run the command above, without any authentication, and the server would gladly accept the incoming mail.
Now, the email is rejected, and the email server only allows authenticated users.

Make gmail's less angry

Ubuntu can't run multiple lines without a special character, thus making the script malfunction, putting the commands on the same line solves the problem and the script can be used on Ubuntu 20.04

documentation changes

According to the wiki (https://wiki.dovecot.org/SSL/DovecotConfiguration):

"From version 2.3, you must specify path to DH parameters file using:
ssl_dh=</path/to/dh.pem"

This actually threw an error something along the lines of: "imap-login: Error: Failed to initialize SSL"

Create user vmail, if not present.
Add 'login' option in dovecot to be able to send emails fro mutt.

The main command for generating DKIM keys (`opendkim`) wasn't installed with just the `opendkim` package.  Added a fix so it would be installed should it not be there the first time `apt` is ran.

opendkim ignores the invalid SOCKET config line, causing smtpd mail send to fail.

$ sudo systemctl status opendkim
systemd[1]: Ignoring invalid environment assignment 'SOCKET=inet:8891@localhost
systemd[1]: ': /etc/sysconfig/opendkim

$ journalctl
postfix/submission/smtpd[17646]: fatal: host/service localhost/8891 not found: Device or resource busy

default /etc/opendkim.conf has KeyTable appear in comment doco and example.
Shouldn't trigger the grep that checks whether this config has been applied.

Deleting all settings starting with o seems rather arbitrary, but I don't know the default configuration on Vultr, so could be warranted :) But I'll be bold and guess the `sed` command and the following lines are supposed to be related?