tcp: send ACKs when requested

When doing Limited Transmit send an ACK when needed by the protocol
processing (like sending ACKs with a DSACK block).

PR: 264257
PR: 263445
PR: 260393
Reviewed by: rscheff@
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D36631

(cherry picked from commit 5ae83e0d871bc7cbe4dcc9a33d37eb689e631efe)