Documented all the argv related code with minor refactoring

Documented all the argv related code with minor refactoring

Added doxygen comments for all the functions in argv.c.

There are some slight refactoring, renaming a few variables to make
their use case more obvious and ensure lines do not break our 80-chars
per line coding style limit.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20200206132103.15977-5-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19377.html