beautify perfdhcp usage code

Merged Andrei Pavel requested to merge 1119-add-dhcp4-releases-to-perfdhcp-2 into master

An extra of #1119 (closed).

A compilation of two commits. You may want to look at them separately:

  • 80259ea7: Sets the usage in the summary in quasi-alphabetical order for easier tracking of flags. Copied the list from the man page which is complete. Flags that were missing before: -g, -J, --scenario
  • b761d8b8: Convert the usage to a raw string literal. It's easier to modify.
Edited by Andrei Pavel

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