close update package before installing; allow remount

Close the update package before invoking the binary, to allow the
installer to unmount /cache if it wants to.  Add a function to allow
remounting of a mount as read-only.

Change-Id: Idfcc96c3da66083295177f729263560be58034e4
diff --git a/install.c b/install.c
index 35ba6ca..40f182d 100644
--- a/install.c
+++ b/install.c
@@ -42,6 +42,7 @@
     const ZipEntry* binary_entry =
             mzFindZipEntry(zip, ASSUMED_UPDATE_BINARY_NAME);
     if (binary_entry == NULL) {
+        mzCloseZipArchive(zip);
         return INSTALL_CORRUPT;
     }
 
@@ -49,11 +50,13 @@
     unlink(binary);
     int fd = creat(binary, 0755);
     if (fd < 0) {
+        mzCloseZipArchive(zip);
         LOGE("Can't make %s\n", binary);
         return 1;
     }
     bool ok = mzExtractZipEntryToFile(zip, binary_entry, fd);
     close(fd);
+    mzCloseZipArchive(zip);
 
     if (!ok) {
         LOGE("Can't copy %s\n", ASSUMED_UPDATE_BINARY_NAME);
@@ -298,6 +301,5 @@
     /* Verify and install the contents of the package.
      */
     int status = handle_update_package(path, &zip);
-    mzCloseZipArchive(&zip);
     return status;
 }