Allow opening read-only ZIP files

[thp]

The function zip_open("somefile.zip", 0, 'r'); fails if somefile.zip is read-only.

To test this, just add a call to chmod(ZIPNAME, S_IRUSR); (needs sys/stat.h included) to the end of test_setup() in test/test_read.c.

The reason for this is that 'r' uses this:

https://github.com/kuba--/zip/blob/114d9d46fdbdad55b37705190a8625f4ab5fa81a/src/zip.c#L847

..which in turn uses this:

https://github.com/kuba--/zip/blob/114d9d46fdbdad55b37705190a8625f4ab5fa81a/src/miniz.h#L5999

An fopen() with mode r+ means that it's opened read-write.

This PR fixes it by using mz_zip_reader_init_file_v2() (which opens in rb mode, not r+b mode) when opening files in 'r' mode.

This fix should help with the downstream issue https://github.com/SuperIlu/DOjS/issues/28