Handle partial data re-sending on ktls/sendfile on FreeBSD
Add a handler for EBUSY sendfile error in addition to
EAGAIN. With EBUSY returned the data still can be partially
sent and user code has to be notified about it, otherwise it
may try to send data multiple times.
Approved by: re (gjb)
Obtained from: OpenSSL (dfcfd17f2818cf520ce6381aed9ec3d2fc12170d)
Sponsored by: Netflix (merging to FreeBSD)