KTLS: Add using_ktls helper variable in ssl3_get_record().

When KTLS receive is enabled, pending data may still be present due to
read ahead. This data must still be processed the same as records
received without KTLS. To ease readability (especially in
consideration of additional checks which will be added for TLS 1.3),
add a helper variable 'using_ktls' that is true when the KTLS receive
path is being used to receive a record.

Obtained from: OpenSSL commit 031132c297e54cbc20404a0bf8de6ed863196399