Enable TLS 1.1 and 1.2, change default DH params

Enable TLS 1.1 and 1.2, change default DH params

AnyEvent uses a built-in DH group defined as 'schmorp1539'
by default, which seems to trigger the bug in [1] for every
attempt of accessing the web GUI using IE11 and TLS1.2. By
switching to a bigger default DH group ('skip2048'), the
bug seems to be gone (or trigger sufficiently rarely).

1: http://engineering.imvu.com/2015/01/27/the-case-of-the-page-cant-be-displayed-intermittent-selenium-test/
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>