rad_counter: Use heredoc for help message

Use Perl's here document syntax for rad_counter's help message and
replace tab escape sequences with spaces to make the source more
readable and easier to modify.
rad_counter: Refine help message

Refine rad_counter help message:

* use executable name without path,
* add program purpose,
* shorten usage summary line for clarity,
* separate arguments and options,
* add full stops to sentences,
* wrap a long line.
dhcpclient: Add a short description to help output

Add a short description of the program's function to the help output of
dhcpclient.
Fix coa_separate to behave differently for proxy reply