fix zerocopy on Solaris

[kulikjak]

Currently, -Z/--zerocopy argument doesn't work on Solaris. HAVE_SENDFILE is defined, but then there is no sendfile implementation to be used, which results in nothing ever being sent:

Accepted connection from 127.0.0.1, port 59470
[  5] local 127.0.0.1 port 5201 connected to 127.0.0.1 port 55152
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   2.00-3.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   3.00-4.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec
.....

This PR send Solaris the Linux way, which fixes the issue.

[bmah888]

Thanks for the PR! We'll take a look, even though Solaris isn't an officially supported OS we've taken compatibility fixes such as this one for Solaris and other UNIX-y platforms in the past.