* rework os.sendfile and add macosx support, and a fallback
implementation for any OS.
* fix sendto compile error
* std.os write functions support partial writes. closes#3443.
* std.os pread / pwrite functions can now return `error.Unseekable`.
* std.fs.File read/write functions now have readAll/writeAll variants
which loop to complete operations even when partial reads/writes
happen.
* Audit std.os read/write functions with respect to Linux returning
EINVAL for lengths greater than 0x7fff0000.
* std.os read/write shim functions do not unnecessarily loop. Since
partial reads/writes are part of the API, the caller will be forced
to loop anyway, and so that would just be code bloat.
* Improve doc comments
* Add a non-trivial test for std.os.sendfile
* Fix std.os.pread on 32 bit Linux
* Add missing SYS_sendfile bit on aarch64
Thanks to the Windows Process Environment Block, it is possible to
obtain handles to the standard input, output, and error streams without
possibility of failure.
Andrew Kelley
Benjamin Feng