Unset got_cancel_cmd after reacting to it, as is already done for got_pause_cmd

Unset got_cancel_cmd after reacting to it, as is already done for got_pause_cmd

Per David Woodhouse (http://lists.infradead.org/pipermail/openconnect-devel/2016-October/004034.html):

> I think it's probably OK to set vpninfo->got_cancel_cmd=0 in the mainloop
> right before calling proto->vpn_close_session.  If we get cancelled
> *again* then we'll give up on that too.

Without this fix, do_https_request() can't be used to close the
session — it interrupts itself as soon as it sees that got_cancel_cmd is
set.

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