Restrict DELPORT to non-active ports only

[csprr]

Possible fix (if wanted) for issue #1806
In "danger" of locking yourself out by removing your active port, this should not be possible (same as to HTTP) in my opinion.

[codecov]

Codecov Report

Merging #1808 (a15ebc5) into master (9cd3605) will decrease coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1808      +/-   ##
==========================================
- Coverage   41.34%   41.26%   -0.09%     
==========================================
  Files         122      122              
  Lines       27578    27581       +3     
  Branches       33       33              
==========================================
- Hits        11402    11380      -22     
- Misses      16151    16176      +25     
  Partials       25       25              

Impacted Files	Coverage Δ
src/ClientCommand.cpp	11.22% <0.00%> (-0.90%)	⬇️
include/znc/Utils.h	65.95% <0.00%> (-6.39%)	⬇️
src/main.cpp	45.37% <0.00%> (-1.20%)	⬇️
src/Threads.cpp	88.75% <0.00%> (-1.19%)	⬇️

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