bug#3770: archive-mode: recognize ZIP files with non-standard "PK00" header

[Peter Jolly]

Some ZIP files have the string "PK00" before the first PK^C^D header.
Apparently this is added by certain archivers when writing to removable
media.

archive-find-type does not currently recognize such files.  The rest of
archive-mode handles them correctly, so I believe they can be supported
with a trivial change: something like

diff --git a/lisp/arc-mode.el b/lisp/arc-mode.el
index cc59d6e..174d205 100644
--- a/lisp/arc-mode.el
+++ b/lisp/arc-mode.el
@@ -716,7 +716,7 @@ archive.
   ;; The funny [] here make it unlikely that the .elc file will be treated
   ;; as an archive by other software.
   (let (case-fold-search)
-    (cond ((looking-at "[P]K\003\004") 'zip)
+    (cond ((looking-at "\\(PK00\\)?[P]K\003\004") 'zip)
          ((looking-at "..-l[hz][0-9ds]-") 'lzh)
          ((looking-at "....................[\334]\247\304\375") 'zoo)
          ((and (looking-at "\C-z")     ; signature too simple, IMHO