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.

PR: 251969
Obtained from: OpenSSL (dfcfd17f2818cf520ce6381aed9ec3d2fc12170d)
Sponsored by: Netflix (merging to FreeBSD)

(cherry picked from commit 9b2f020c14af71a2606012143432dd717c7cf90e)