The current implementation placed the dependency on
github.com/chzyer/readline in pprof.go instead of placing
it in the default ui just because the go repository that
vendors pprof may not want the dependency.

I evaluated another option (golang.org/x/crypto/ssh/terminal)
for the Go repository and there exist subtle differences
in the behavior. Inconsistency in UI is undesirable so golang
decided to vendor the chzyer readline as well for now.

Update golang/go#14041

The upstream pprof implements the readline feature using
the github.com/chzyer/readline package in its pprof.go main.

It would be ideal to use the same readline support package as
the upstream for better user experience and code maintenance.
However, bringing in third-party packages requires more work
than I envisioned (e.g. clean up the vendored code to meet the
expected standard - iow don't break builders).

As a result, this change implements the similar feature
for the pprof command included in the go distribution
(cmd/pprof/pprof.go) using golang.org/x/crypto/ssh/terminal
for now.

Auto-completion is not yet supported (same in the upstream).

The feature is enabled only in linux, windows, darwin, and
only when terminal support is available.

This change brings in new vendored packages,
golang.org/x/crypto/ssh/terminal and
golang.org/x/sys/{unix,windows}.

For #14041

Change-Id: If4a790796acf2ab20f7e81268b9d9354c5a5cd2b
Reviewed-on: https://go-review.googlesource.com/112436
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>