Don't let specific TLS validation flags be set

We have a method in the IMAP client that allows the caller to set the
TLS validation flags. A problem with this is that we expose the TLS
implementation, and by doing that we can't easily switch implementation
in the future if we would like to. It also doesn't give much value. An
application would either use full validation or no validation, not
something in between. Remove the possibility to set specific validation
flags. Only allow validation to be enabled or disabled. This also
removes the need to include gio.h in the public header.

It is possible that setting specific flags might be useful to some
applications. At that point we might want to reintroduce the feature by
creating a wrapper for the validation flags.

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