Rework output redirection in Spack.

- Simplify interface to log_output. New interface requires only one
  context handler instead of two.  Before:

      with log_output('logfile.txt') as log_redirection:
           with log_redirection:
               # do things ... output will be logged

  After:

      with log_output('logfile.txt'):
          # do things ... output will be logged

  If you also want the output to be echoed to ``stdout``, use the
  `echo` parameter::

      with log_output('logfile.txt', echo=True):
          # do things ... output will be logged and printed out

  And, if you just want to echo *some* stuff from the parent, use
  ``force_echo``:

      with log_output('logfile.txt', echo=False) as logger:
          # do things ... output will be logged

          with logger.force_echo():
              # things here will be echoed *and* logged

  A key difference between this and the previous implementation is that
  *everything* in the context handler is logged.  Previously, things like
  `Executing phase 'configure'` would not be logged, only output to the
  screen, so understanding phases in the build log was difficult.

- The implementation of `log_output()` is different in two major ways:

  1. This implementation avoids race cases by using only one pipe (before
     we had a multiprocessing pipe and a unix pipe).  The logger daemon
     stops naturally when the input stream is closed, which avoids a race
     in the previous implementation where we'd miss some lines of output
     because the parent would shut the daemon down before it was done
     with all output.

  2. Instead of turning output redirection on and off, which prevented
     some things from being logged, this version uses control characters
     in the output stream to enable/disable forced echoing.  We're using
     the time-honored xon and xoff codes, which tell the daemon to echo
     anything between them AND write it to the log.  This is how
     `logger.force_echo()` works.

- Fix places where output could get stuck in buffers by flushing more
  aggressively.  This makes the output printed to the terminal the same
  as that which would be printed through a pipe to `cat` or to a file.
  Previously these could be weirdly different, and some output would be
  missing when redirecting Spack to a file or pipe.

- Simplify input and color handling in both `build_environment.fork()`
  and `llnl.util.tty.log.log_output()`.  Neither requires an input_stream
  parameter anymore; we assume stdin will be forwarded if possible.

- remove `llnl.util.lang.duplicate_stream()` and remove associated
  monkey-patching in tests, as these aren't needed if you just check
  whether stdin is a tty and has a fileno attribute.

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