postfwd defaults: keep lines with the wanted value, no reload if unchanged

- Never comment out a line that already has the wanted value, even if a
  differing setting of the same variable precedes it.
- update_postfwd_defaults.sh: leave postfwd alone if nothing changed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

install_postfwd.sh

@@ -120,19 +120,30 @@ set_postfwd_default() {
    _tmp_file="$(mktemp)"
    awk -v var="$_var" -v val="$_val" '
       function unq(s) {
-         gsub(/^[ \t]+|[ \t]+$/, "", s)
+         gsub(/^[ \t\r]+|[ \t\r]+$/, "", s)
          if (s ~ /^".*"$/ || s ~ /^\047.*\047$/) s = substr(s, 2, length(s) - 2)
          return s
       }
-      $0 ~ "^[ \t]*" var "=" {
-         cur = $0 ; sub("^[ \t]*" var "=", "", cur)
-         if (! done && unq(cur) == unq(val)) { print ; done = 1 ; next }
-         print "#" $0
-         if (! done) { print var "=" val ; done = 1 }
-         next
+      function is_set(s) { return s ~ ("^[ \t]*" var "=") }
+      function is_wanted(s) {
+         sub("^[ \t]*" var "=", "", s)
+         return unq(s) == unq(val)
+      }
+      # - Read the whole file first: a line with the wanted value must stay
+      # - untouched, no matter which other settings of VAR precede it.
+      {
+         line[NR] = $0
+         if (is_set($0) && is_wanted($0)) found = 1
+      }
+      END {
+         for (i = 1; i <= NR; i++) {
+            if (! is_set(line[i])) { print line[i] ; continue }
+            if (is_wanted(line[i])) { print line[i] ; continue }
+            print "#" line[i]
+            if (! found) { print var "=" val ; found = 1 }
+         }
+         if (! found) print var "=" val
       }
-      { print }
-      END { if (! done) print var "=" val }
    ' "$postfwd_default_file" > "$_tmp_file" 2> "$log_file"
 
    if [[ $? -ne 0 ]] ; then