Merge branch 'login_signal' of https://github.com/calavera/docker into login_signal

* 'login_signal' of https://github.com/calavera/docker:
  Use flag.StringVar to capture the command line flags.
  Fix syscall name.
  Remove unused imports.
  Simplify term signal handler.
  Add the ISIG syscall back to not kill the client withing a shell with ctrl+c.
  Print a new line after getting the password from stdin.
  Exit if there is any error reading from stdin.
  Stop making a raw terminal to ask for registry login credentials.
  Allow to generate signals when termios is in raw mode.
  Use a more idiomatic syntax to capture the exit.
  Exit from `docker login` on SIGTERM and SIGINT.