With --user, enter username in all forms, not just the first

With --user, enter username in all forms, not just the first

Until now, the -u/--user=USERNAME option has caused the OpenConnect CLI to
automatically fill the username only in the *first* form where there is a
match field.  This patch causes it to fill the username repeatedly
(including when a form is repeated due to an authentication failure).

As described by @DimitriPapadopoulos in #267:

> In many cases, I expect the authentication failure to be caused by an
> incorrect password, not an incorrect username
>
> Having to [re]enter the username, when it has already been specified from
> the command line or worse in a config file, is an annoyance.  I suggest
> openconnect [re]prompts for the username only when it has been entered
> interactively from the start.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>