This fixes several minor issues with wr_mon. It changes the term.c interface,
bur wr_mon is the only user of it and term.h.

  - term: support b/w mode (no colors)
  - term: simplify termios setting, and don't do it on non-tty files
  - term: term_poll now does poll, so user input is handled quickly
  - term: exit on ctrl-C
  - term: don't fflush() on every cprintf
  - wr_mon: accept "-b" for "black and white"
  - wr_mon: fix buffering of stdout, fflush the whole screenshot
      (this results in much less flashing in the terminal window)
  - wr_mon: use \n where possible, not calculated positions
      (this simplified future addition of dynamic interface listings)
  - wr_mon: shrink output vertically, so in no-servo mode it fits 80x25
  - wr_mon: use term_poll, not explicit sleep
  - wr_mon: don't clear the screen at exit time
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>