[PATCHv2 0/7] getpass refactoring

This is a re-roll of the earlier "echo usernames as they are typed"
series here:

  http://article.gmane.org/gmane.comp.version-control.git/185970

It turns out that getpass() isn't even really good at getting passwords.
See patch 5/7 for details. This series replaces it entirely if you set
HAVE_DEV_TTY (otherwise, we continue to fallback to getpass()).

  [1/7]: imap-send: avoid buffer overflow
  [2/7]: imap-send: don't check return value of git_getpass
  [3/7]: move git_getpass to its own source file
  [4/7]: refactor git_getpass into generic prompt function
  [5/7]: add generic terminal prompt function
  [6/7]: prompt: use git_terminal_prompt
  [7/7]: credential: use git_prompt instead of git_getpass

It should be applied on top of the jk/credentials topic.

-Peff