cmd/pprof: add readline support

[valyala]

Currently go tool pprof in interactive mode doesn't support readline features such as command history, ctrl-r, tab-completion, etc. It would be great to add such functionality.

Basic integration is quite easy - see the gist containing PoC readline patch for go tool pprof. It is based on pure go implementation of readline.

[ALTree]

upstream issue: google/pprof/issues/57

Unfortunately it seems that the pprof maintainers are reluctant to add an external dependency to the tool.

[hyangah]

The upstream pprof maintainer @aalexand proposed to utilize the golang.org/x/crypto/ssh/terminal package for readline functionality.
Go repo already vendored some packages from x/crypto (https://github.com/golang/go/tree/master/src/vendor/golang_org/x/crypto).
Is it okay to add the ssh/terminal package there?

[gopherbot]

Change https://golang.org/cl/112436 mentions this issue: cmd/pprof: provides readline feature

[hyangah]

cl/112436 implements the feature requested here. I was ok'd about vendoring x/crypto/ssh/terminal but found it depends on some packages in x/sys as well. They bring in a lot more files than I hoped and I am currently fighting for all kinds of build/vet errors discovered while vendoring. Already 1.11 dev tree is closed and I am afraid it's too late.