Improve join_by to support multi character separator

bashopts.sh

@@ -387,9 +387,12 @@ bashopts_math_min() {
 
 # join array element
 bashopts_join_by() {
-    local IFS="$1"
-    shift || bashopts_log C "Usage: bashopts_join_by <character> [elt1 [elt2...]]"
-    echo "$*"
+    local sep="$1"
+    shift || bashopts_log C "Usage: bashopts_join_by <separator> [elt1 [elt2...]]"
+    printf "%s" "$1"
+    test $# -gt 1 || return 0
+    shift
+    printf "$sep%s" "$@"
 }
 
 # dump an option value by its name