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?