Add AUTH command to smtp service

[fvdveen]

Added the AUTH command with the options PLAIN, LOGIN and CRAM-MD5 to the
smtp service.

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[sammynx]

If I run telnet 127.0.0.1 <configured-port> there is no welcome message and after EHLO command the connection is closed.

[sammynx]

Also could you put a configuration sample in smtp.go. See services/ldap/ldap.go for an example.

[sammynx]

In smtp_test.go func TestMain() could you cleanup the storage when testing is done.

and do a rebase on master. thnx

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@99dcbf3). Click here to learn what that means.
The diff coverage is 53.71%.

@@            Coverage Diff            @@
##             master     #386   +/-   ##
=========================================
  Coverage          ?   41.76%           
=========================================
  Files             ?       78           
  Lines             ?     8930           
  Branches          ?        0           
=========================================
  Hits              ?     3730           
  Misses            ?     4757           
  Partials          ?      443

Impacted Files	Coverage Δ
services/smtp/server.go	86.66% <ø> (ø)
services/smtp/smtp.go	83.07% <100%> (ø)
services/smtp/conn.go	50.71% <51.72%> (ø)

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[nl5887]

I think it is not that interesting to check for a pre-existing list of users and secrets, but more what users and secrets are being used.

[Neustradamus]

Please do not add unsecure and old CRAM-MD5, LOGIN mechanisms.

• https://tools.ietf.org/html/draft-ietf-sasl-crammd5-to-historic-00
• https://tools.ietf.org/html/draft-zeilenga-luis140219-crammd5-to-historic-00

LOGIN has been replaced by PLAIN but PLAIN can not be used without a secure connection.