Allow specification of an "alternative secret" field for GP login form(s), instead...

Allow specification of an "alternative secret" field for GP login form(s), instead of 'passwd'.

The alternative field is specified by appending :FIELDNAME to the URL path, or --usergroup.

Known use cases:

	openconnect --protocol=gp vpn.bigcorp.com --usergroup=gateway:prelogin-cookie
	openconnect --protocol=gp vpn.bigcorp.com --usergroup=portal:portal-userauthcookie

Signed-off-by: Daniel Lenski <dlenski@gmail.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>