Improve loader passwords:

1. Be clear about which password is being requested
2. Remove extraneous whitespace between the prompt and the cursor
3. Move the twiddle to where the prompt is, instead of two characters to the right
4. Fix erasing the 'incorrect password' message when retrying; previously it was erased partially
5. Remove the unneeded exclamation mark

Reviewed by: kevans
Approved by: re (gjb)
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17236