Add support for loading PKCS#12 bundles.

Add support for loading certificates and keys from PKCS#12 bundles
(also known as pfx files).

Task-number: QTBUG-1565

[ChangeLog][QtNetwork][QSslSocket] Support for loading PKCS#12
bundles was added. These are often used to transport keys and
certificates conveniently, particularly when making use of
client certificates.

Change-Id: Idaeb2cb4dac4b19881a5c99c7c0a7eea00c2b207
Reviewed-by: Daniel Molkentin <daniel@molkentin.de>