These are just based on the read/pread{,64} implementations with the
function calls and error messages adjusted as appropriate. The only
difference is that the buffer parameters are const.
Change-Id: Ida1597a903807f583f230d74bcedffdb7b24fcf6
A __size_mul_overflow utility is used to take advantage of the checked
overflow intrinsics in Clang and GCC (>= 5). The fallback for older
compilers is the optimized but less than ideal overflow checking pattern
used in OpenBSD.
Change-Id: Ibb0d4fd9b5acb67983e6a9f46844c2fd444f7e69
