Don't accept invalid certificates by default

We always accept the certificate even if it failed validation. This is
not a good default. The user should be able to inspect the certificate,
the reason why it didn't validate and be given the option to accept or
reject it.

We won't do that now but we will take a step in that direction. Reject
invalid certificates by default and add a command line option to accept
it.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>