Add support for storing user data in a solv file

Add support for storing user data in a solv file

Userdata can be arbritrary (binary)data with a maximum size of
65535 bytes. It can be read without reading the complete
solv file, but do not forget to rewind the fp after reading
the user data.

New functions:
void
    void repowriter_set_userdata(Repowriter *writer, const void *data, int len)
    int solv_read_userdata(FILE *fp, unsigned char **datap, int *lenp)