In general, dumb terminal indicates terminal with limited capability.
It may provide no support for special character sequences, e.g., no
handling of ANSI escape sequences. Its input/output handling behavior
may deviate from what's described in termios or terminfo. E.g., in
the shell in emacs, even after successfully setting the terminal to
raw mode, the terminal behaves as if it's still operating in canonical
mode since emacs is doing input processing first.

Readline support can be broken in various ways in dumb terminal mode,
so we want to disable readline or advanced UI features. The easiest
way to detect dumb terminal is to check the environment variable "TERM".

Fixes #26254

Change-Id: I6b652eb555bc03b84405aae08b0b25d111fbb8b0
Reviewed-on: https://go-review.googlesource.com/122879Reviewed-by: Ian Lance Taylor <iant@golang.org>