Replace missing getpass()

Replace missing getpass()

getpass() is traditional Unix, but has been withdrawn from POSIX.  As
such, it may be missing.  Check for that, and provide ersatz.  It's
not a real replacement, because it doesn't do the special magic
getpass() is supposed to do: read from /dev/tty without echo.

This bypasses our existing getpass() for Windows.  In contrast to the
portable getpass(), the Windows one tries to turn off echo, but that
doesn't work for me (MinGW & Wine).  Remove it.