tests/sendfile_helper: support remote connection

In order to use the sendfile_helper program in a pf test script that
requires non-loopback interfaces, add functionality to sendfile with a
TCP socket that is connected to a remote host.
The behavior for unix sockets and TCP loopback sockets is unchanged.

Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D58041