Revert "Revert "od -c => od -tc: od -c is an XSI extension..."

This reverts commit 0e70f7a57e.

There is no reason to revert this change.

In #2922, I only disagreed with the commit message suggesting that
  LC_CTYPE=C od -t c    is   equivalent to   od -c

The only documented differences are that -tc is required to be
influenced by -N and -j, while -c is not, and that -c is required to
only support a subset of the backslash sequences that -tc should
support.

tests/shtest

@@ -517,23 +517,23 @@ if $test_no_color && command -v script >/dev/null 2>&1; then
 
   faketty $JQ_NO_B -n . > $d/color
   printf '\033[0;90mnull\033[0m\r\n' > $d/expect
-  od -c $d/expect
-  od -c $d/color
+  od -tc $d/expect
+  od -tc $d/color
   cmp $d/color $d/expect
   NO_COLOR= faketty $JQ_NO_B -n . > $d/color
   printf '\033[0;90mnull\033[0m\r\n' > $d/expect
-  od -c $d/expect
-  od -c $d/color
+  od -tc $d/expect
+  od -tc $d/color
   cmp $d/color $d/expect
   NO_COLOR=1 faketty $JQ_NO_B -n . > $d/color
   printf 'null\r\n' > $d/expect
-  od -c $d/expect
-  od -c $d/color
+  od -tc $d/expect
+  od -tc $d/color
   cmp $d/color $d/expect
   NO_COLOR=1 faketty $JQ_NO_B -Cn . > $d/color
   printf '\033[0;90mnull\033[0m\r\n' > $d/expect
-  od -c $d/expect
-  od -c $d/color
+  od -tc $d/expect
+  od -tc $d/color
   cmp $d/color $d/expect
 fi