unzip: Handle non-seekable input ports.

Before this commit:
  - unzip would attempt to seek the input port via file-position in some cases

After this commit:
  - unzip reads and discards bytes as needed to skip forward

pkgs/racket-pkgs/racket-test/tests/file/unzip.rkt

@@ -1,6 +1,7 @@
 #lang racket/base
 (require file/unzip
          racket/runtime-path
+         racket/port
          tests/eli-tester)
 
 (define-runtime-path unzip-me.zip "unzip-me.zip")
@@ -24,6 +25,8 @@
 (define (run-tests)
   (test-with-unzip unzip-me.zip)
   (call-with-input-file* unzip-me.zip test-with-unzip)
+  (call-with-input-file* unzip-me.zip
+                         (lambda(in_port) (test-with-unzip (input-port-append #f in_port))))
   (test-with-unzip-entry))
 
 (provide tests)

racket/collects/file/unzip.rkt

@@ -217,7 +217,7 @@
               ;; is immediately followed by a data descriptor
               (if (bitwise-bit-set? bits 3)
                   (skip-bytes 12 in)
-                  (file-position in (+ mark compressed))))))
+                  (skip-bytes (- (+ mark compressed) (file-position in)) in)))))
       (void))))
 
 ;; find-central-directory : input-port nat -> nat nat nat