transfer: avoid unnecessary timeout event when waiting for 100-continue

The commit 9dd85bce unintentionally changed the way we compute the time
spent waiting for 100-continue.  In particular, when using a SSL client
certificate, the time spent by SSL handshake was included and could
cause the CURL_TIMEOUT_EXPECT_100 timeout to be mistakenly fired up.

Bug: https://bugzilla.redhat.com/767490
Reported by: Mamoru Tasaka