openvpn_PRF: Change API to use size_t for lengths

openvpn_PRF: Change API to use size_t for lengths

Basically all users already wanted that anyway. And most
of the library functions also take size_t nowadays.

Change-Id: Ic88cd6e143bc48cab3c9ebb7c7007513803bd199
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: MaxF <max@max-fillinger.net>
Message-Id: <20250911201719.25773-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59232185/
URL: https://gerrit.openvpn.net/c/openvpn/+/1135
Signed-off-by: Gert Doering <gert@greenie.muc.de>