sshd filter enhancements (gh-2239)

[sebres]

introduces new failregex-flag tag <F-MLFGAINED> signaled that the access to service was gained (ATM used similar to <F-NOFAIL>, but it does not add the log-line to matches);

filter.d/sshd.conf: extended with new rules (#2239):

• Disconnecting ...: Change of username or service not allowed
• Disconnected from ... [preauth] (extra/aggressive mode only)

[codecov-io]

Codecov Report

Merging #2279 into 0.10 will decrease coverage by <.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             0.10    #2279      +/-   ##
==========================================
- Coverage   95.77%   95.76%   -0.01%     
==========================================
  Files          76       76              
  Lines       13272    13278       +6     
  Branches     2097     2099       +2     
==========================================
+ Hits        12711    12716       +5     
  Misses        293      293              
- Partials      268      269       +1

Impacted Files	Coverage Δ
fail2ban/server/filter.py	95.85% <88.88%> (-0.11%)	⬇️

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[coveralls]

Coverage decreased (-0.007%) to 97.716% when pulling 0ac5c89 on sebres:sshd-filter-gh-2239 into 657b147 on fail2ban:0.10.