scripts: add wrapper to update Postfix configuration safely (#3484)

The new function can

1. update/append
2. update/prepend
3. initialize if non-existent

options in `/etc/postfix/main.cf` in a safe and secure manner. When the
container is improperly restarted, the option is not applied twice.

---

Co-authored-by: Casper <casperklein@users.noreply.github.com>
Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

target/scripts/helpers/utils.sh

@@ -4,6 +4,15 @@ function _escape() {
   echo "${1//./\\.}"
 }
 
+# Replaces a string so that it can be used inside
+# `sed` safely.
+#
+# @param ${1} = string to escape
+# @output     = prints the escaped string
+function _escape_for_sed() {
+  sed -E 's/[]\/$*.^[]/\\&/g' <<< "${1:?String to escape for sed is required}"
+}
+
 # Returns input after filtering out lines that are:
 # empty, white-space, comments (`#` as the first non-whitespace character)
 function _get_valid_lines_from_file() {