tcp: commonize check for more data to send, style changes

Use SEQ_SUB instead of a plain subtraction, for an implict
type conversion and prevention of a possible overflow.
Use curly brackets in stacked if statements throughout.
Use of the ? operator to enhance readability when clearing
the FIN flag in tcp_output().

None of the above change the function.

Reviewed By: tuexen, cc, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43539