os/exec: use "pfiles" for fd debugging on illumos

On illumos (and Solaris) systems, the native "pfiles" tool provides the
best information about open file descriptors for a process:

    https://illumos.org/man/1/pfiles

Use that instead of "lsof" when debugging file descriptor leaks.

Updates #42431.

Change-Id: If1250c4e6c9e8adbd076495a09fb1ce63abcc68b
Reviewed-on: https://go-review.googlesource.com/c/go/+/268019


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>