beautify perfdhcp usage code

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.