Replace, don't remove control characters

Make it more consistent with other similar functions.  Note that one can
always get the old behavior by setting $COERCE__REPCHAR to empty string.

src/include-sh/coerce.sh

 
 coerce__noctl() {
     #
-    # Remove non-printable characters
+    # Replace non-printable characters with $COERCE__REPCHAR
     #
-    perl -CS -Mutf8 -pe "s|[^[:graph:] \t\n]||g"
+    perl -CS -Mutf8 -pe "s|[^[:graph:] \t\n]|$COERCE__REPCHAR|g"
 }
 
 coerce__nofdraw() {