Add openconnect_set_external_browser_callback() and defaults

Add openconnect_set_external_browser_callback() and defaults

There is a little bit of duplication here since the *library* will spawn
the default (xdg-open) and the application (main.c) has basically the same
code, but that's because we don't want to *require* that the app register
the callback. With suitable default behaviour from the library, it will
work even with existing versions of the NM auth-dialog or other GUI tools.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>