Remove "invalid argument" statuses

Remove "invalid argument" statuses

This patch removes the only remaining "invalid argument" status, which
parse_long_opt() sets when the length of the name of a long option is
greater than the size of its temporary buffer.

To deal with this, `struct argpar_iter` contains a temporary buffer to
contain the long option name in parse_long_opt(). This temporary buffer
only grows.

`test_argpar.c` contains a new test which exercise the parsers with a
700-character, hipster-themed long option name.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ia5adcfc2bdd978d81841a19ceea4a02f44559eed