Add funopen() function
This is a wrapper over the glibc fopencookie() function. We diverge from the FreeBSD, OpenBSD and DragonFlyBSD declarations, because seekfn() there wrongly uses fpos_t, assuming it's an integral type, and any code using that on a system where fpos_t is a struct (such as GNU-based systems or NetBSD) will fail to build. In which case, as the code has to be modified anyway, we might just as well use the correct declaration.
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