Improve linenoise.c compatability.

+ Don't flush characters when switching into and out of raw mode. This
avoids eating characters that follow '\n'. (Such characters can occur
when pasting multiple line input, or when scripts are driving input.)

+ Try to be slightly cleverer about calculating the length of the prompt,
so that prompts with embedded '\n' characters are handled OK. This is
an area that really needs to be replaced with a query of the cursor
position from the terminal.

+ As a hack, just assume the screen is very wide if we don't know how
wide it is. This allows dexpropt to work correctly. (It was getting
confused by the editing commands emitted when the end-of-line was reached.)

Change-Id: I988dd0f0bceb22b298e915be0dde085c9358ef66