Instead of manual string splitting (EW!), use QDateTime parsing.
Moreover, X.509 certificates *must* have a valid start/end date.
In case of parsing failure, reject the certificate. An autotest
for this last case is coming in a separate patch.
Change-Id: I934bf9e6a4a92e4befdb3b0f9450f76f67bad067
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>