patch 9.0.1834: Some problems with xxd coloring

Problem:  Some problems with xxd coloring
Solution: Fix the following problems:

* Support colored output on Windows.
  SetConsoleMode() is required to enable ANSI color sequences.
* Support "NO_COLOR" environment variable.
  If "NO_COLOR" is defined and not empty, colored output should be
  disabled.
  See https://no-color.org/
* "-R" should only accept "always", "never" or "auto" as the parameter.
* Adjust help and documentation. "-R" cannot omit the parameter. Remove
  surrounding brackets.

Related #12131
closes: #12997
closes: #12991
closes: #12986

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: K.Takata <kentkt@csc.jp>
diff --git a/runtime/doc/xxd.1 b/runtime/doc/xxd.1
index 9ae132b..e52ef88 100644
--- a/runtime/doc/xxd.1
+++ b/runtime/doc/xxd.1
@@ -135,9 +135,9 @@
 particular column layout. Additional whitespace and line breaks are allowed
 anywhere.
 .TP
-.IR \-R " "[WHEN]
+.IR \-R " " when
 In output the hex-value and the value are both colored with the same color depending on the hex-value. Mostly helping to differentiate printable and non-printable characters.
-.I WHEN
+.I \fIwhen\fP
 is
 .BR never ", " always ", or " auto .
 .TP