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>

main.c

@@ -2614,8 +2614,7 @@ static int process_auth_form_cb(void *_vpninfo,
 			if (username &&
 			    (!strncasecmp(opt->name, "user", 4) ||
 			     !strncasecmp(opt->name, "uname", 5))) {
-				opt->_value = username;
-				username = NULL;
+				opt->_value = strdup(username);
 			} else {
 				opt->_value = saved_form_field(vpninfo, form->auth_id, opt->name);
 				if (!opt->_value)