scripts: add script to convert multiline comments into 4-line format

Since we're adding checkpatch rules to enforce 4-line multiline comment
format, i.e. with lone /* and */, this script can be run on existing
code so that the comment style does not become inconsistent within a
file.

The alternative to awk-in-a-shell-script could be Perl, which also
supports -i directly, but a2p seems to have bitrotten and I didn't quite
feel like writing this twice...

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/fix-multiline-comments.sh

@@ -0,0 +1,62 @@
+#! /bin/sh
+#
+# Fix multiline comments to match CODING_STYLE
+#
+# Copyright (C) 2018 Red Hat, Inc.
+#
+# Author: Paolo Bonzini
+#
+# Usage: scripts/fix-multiline-comments.sh [-i] FILE...
+#
+# -i edits the file in place (requires gawk 4.1.0).
+#
+# Set the AWK environment variable to choose the awk interpreter to use
+# (default 'awk')
+
+if test "$1" = -i; then
+  # gawk extension
+  inplace="-i inplace"
+  shift
+fi
+${AWK-awk} $inplace 'BEGIN { indent = -1 }
+{
+    line = $0
+    # apply a star to the indent on lines after the first
+    if (indent != -1) {
+        if (line == "") {
+            line = sp " *"
+        } else if (substr(line, 1, indent + 2) == sp "  ") {
+            line = sp " *" substr(line, indent + 3)
+        }
+    }
+
+    is_lead = (line ~ /^[ \t]*\/\*/)
+    is_trail = (line ~ /\*\//)
+    if (is_lead && !is_trail) {
+        # grab the indent at the start of a comment, but not for
+        # single-line comments
+        match(line, /^[ \t]*\/\*/)
+        indent = RLENGTH - 2
+        sp = substr(line, 1, indent)
+    }
+
+    # the regular expression filters out lone /*, /**, or */
+    if (indent != -1 && !(line ~ /^[ \t]*(\/\*+|\*\/)[ \t]*$/)) {
+        if (is_lead) {
+            # split the leading /* or /** on a separate line
+            match(line, /^[ \t]*\/\*+/)
+            lead = substr(line, 1, RLENGTH)
+            match(line, /^[ \t]*\/\*+[ \t]*/)
+            line = lead "\n" sp " *" substr(line, RLENGTH)
+        }
+        if (is_trail) {
+            # split the trailing */ on a separate line
+            match(line, /[ \t]*\*\//)
+            line = substr(line, 1, RSTART - 1) "\n" sp " */"
+        }
+    }
+    if (is_trail) {
+        indent = -1
+    }
+    print line
+}' "$@"