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

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 <[email protected]>