Fix 'zfs send -D' segfault

Sending pools with dedup results in a segfault due to a Solaris
portability issue. Under Solaris the pipe(2) library call
creates a bidirectional data channel. Unfortunately, on Linux
pipe(2) call creates unidirection data channel. The fix is to
use the socketpair(2) function to create the expected
bidirectional channel.

Seth Heeren did the original leg work on this issue for zfs-fuse.
We finally just rediscovered the same portability issue and
dfurphy was able to point me at the original issue for the fix.

Closes #268