Increase max input for password/bootlock_password from 16 to 255.

Increase max input for password/bootlock_password from 16 to 255.
When taking user input, don't show asterisks as the user types
but instead spin a twiddle. Implement Ctrl-U to clear user input.
If the buffer is empty, either because the user has yet to type
anything, presses Ctrl-U at any time, or presses backspace enough
to end in an empty buffer, the twiddle is erased to provide feed-
back to the user.

MFC after:	3 days
X-MFC-to:	stable/10 stable/9