util/progress: no progress reporting in dumb terminals

util/progress: no progress reporting in dumb terminals

cargo should not assume that all terminals have direct access to
the terminal.  Dumb terminals are those that can interpret only a
limited number of control codes (CR, LF, &c.) and the escape codes
used by the progress bar breaks output in these by asserting control
over the cursor position to draw a bar.

A dumb terminal is identified by the TERM output variable being set to
"dumb".  This adds a direct check for this in src/cargo/util/progress.rs
because TERM=dumb does not imply the same as the -q flag.