os/exec: use "pfiles" for fd debugging on illumos
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go