- Let file:is-tty always take one argument.
- Revert change to eval.ByteOutput.
- Make sys.IsATTY take a FD instead, to avoid the need to use os.NewFile. Using
os.NewFile can cause the Go runtime to start polling the file, which
interferes with Elvish's terminal reader.
Non-portable ones are moved into new packages pkg/sys/e{unix windows}. The "e"
prefix is needed to avoid conflict with packages under golang.org/x/sys/ and can
mean "extra".