Add Shell Completion script generation

`dnsutils shell-completion <shell>` will generate a shell completion
script for the specified shell (bash or zsh). If no shell is specified,
the script will be generated for the current shell, using `$SHELL`.

commands/completion-scripts/completion.bash.gotmpl

@@ -0,0 +1,34 @@
+#!/bin/bash
+
+: "{{.App.Name}}"
+
+# Macs have bash3 for which the bash-completion package doesn't include
+# _init_completion. This is a minimal version of that function.
+_dnscontrol_init_completion() {
+  COMPREPLY=()
+  _get_comp_words_by_ref "$@" cur prev words cword
+}
+
+_dnscontrol() {
+  if [[ "${COMP_WORDS[0]}" != "source" ]]; then
+    local cur opts base words
+    COMPREPLY=()
+    cur="${COMP_WORDS[COMP_CWORD]}"
+    if declare -F _init_completion >/dev/null 2>&1; then
+      _init_completion -n "=:" || return
+    else
+      _dnscontrol_init_completion -n "=:" || return
+    fi
+    words=("${words[@]:0:$cword}")
+    if [[ "$cur" == "-"* ]]; then
+      requestComp="${words[*]} ${cur} --generate-bash-completion"
+    else
+      requestComp="${words[*]} --generate-bash-completion"
+    fi
+    opts=$(eval "${requestComp}" 2>/dev/null)
+    COMPREPLY=($(compgen -W "${opts}" -- ${cur}))
+    return 0
+  fi
+}
+
+complete -o bashdefault -o default -o nospace -F "_dnscontrol" "{{.App.Name}}"