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:

zip/src/zip.c

Line 847 in 114d9d4

if (!mz_zip_reader_init_file_v2_rpb(

..which in turn uses this:

zip/src/miniz.h

Line 5999 in 114d9d4

pFile = MZ_FOPEN(pFilename, "r+b");

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 SuperIlu/DOjS#28

[kuba--]

Thanks