Implement `getpasswd`, fix up github two-factor authentication

[kmsquire]

#5252 #6668 added support for github two-factor authentication. However, the implementation is a bit ugly: in the worst case, the user has to enter her password 4 times(!).

The nicest way to handle this would probably be to implement a getpasswd (or getpass) function, which reads in a password without echoing, and then passes that onto curl via curl -K - (which reads a config file from stdin).

[stevengj]

The Python getpass function may be helpful. There is also getpass.c from glibc, and a thread on stackoverflow.

(We should certainly be able to put together a pure-Julia implementation, but it would be good to take a quick look at other implementations in case there are subtleties.)

[stevengj]

Note that we shouldn't keep the password in memory longer than necessary; probably better to be paranoid and to overwrite the string buffer with zeros when we are done; one also needs to be a bit careful in the getpass implementation to avoid making extra copies of the data.

[StefanKarpinski]

@Keno, what's the best way to do prompting with the REPL? I'm assuming we don't want a new REPL mode for this.

[Keno]

Well, we should implement a simple "prompt" function, but basically what you do is construct a Prompt object and then pass it to prompt! with the currently active terminal.

[StefanKarpinski]

Hmm. I'm messing with this and my need to refactor is getting strong...

[quinnj]

How does this differ from readline(STDIN)? Or is this on the repl side of things, I guess? In any case, does this make it easier to read from the prompt in IJulia?

[StefanKarpinski]

It's quite possible that STDIN is coming from something other than the terminal. Ideally, this should be nicely integrated with our line editing and allow backspace to work, etc.

[stevengj]

readline(STDIN) works in IJulia, but that is about it; IPython is fairly limited in how much we can emulate standard input.

However, password input requires a bit different interface, because you don't want the password to appear as it is typed. And sending passwords via IJulia requires even more thought about security; I wouldn't recommend it for now.

[tkelman]

I think this should be reopened at PkgDev.jl. @wildart @jakebolewski how much of this is already functioning?

[stevengj]

A getpasswd function might be more generally useful than just PkgDev.