Py_UNICODE_strrchr.patch: Create Py_UNICODE_strrchr() function. It will be used for zipimport to work on unicode paths instead of bytes paths.

Antoine noticed that the input string is const whereas the output string is not const, which is unusual. I copy/pasted Py_UNICODE_strchr() prototype.

I suppose that const input and non const input is required to be able to use the function on const strings. The GNU libc uses the same strchr() prototype in its C version of string.h. In the C++ version of the header, it defines the strchr() twice: once with const input and output, once with non const input and ouput. The right solution is the C++ way, but C doesn't support polymophism.