Update fmemopen from NetBSD.

Update fmemopen from NetBSD.

NetBSD's version of fmemopen() was more correct than our own wrt error
handling and handling seeks at the tail of buffers. We now pass the ATF
fmemopen test set, with the exception of test08; that problem is related to
malloc return for SIZE_MAX requests.

Changes:
* Modified NetBSD's fmemopen to work with our libc's private FILE struct
* Modified fmemopen_read() to use bcopy() rather than a byte-at-a-time loop