Do not require stdout and stderr to have FDFLAGS_APPEND set

[caspervonb]

Currently stdout and stderr expected to have FDFLAGS_APPEND set the fdstat_get test (https://github.com/khronosproject/wasi-test/blob/master/src/bin/wasi_fd_fdstat_get.rs)

This is probably wrong and most likely shouldn't be required.
It was just done because it was common behaviour shared between Node, Deno and Wasmer when I wrote those test cases.

[caspervonb]

One of the probably wrong assumptions I brought up in yesterday's call @sunfishcode

[sunfishcode]

Right now, WASI is largely deferring to POSIX in these matters, and in POSIX, O_APPEND (the analog of FDFLAGS_APPEND) is only set when stdout and stderr are attached to a file descriptor opened with O_APPEND. When connected to a terminal, for example, it shouldn't be set. When redirected to a file with > in a typical Unix shell, it shouldn't be set. When redirected to a file with >> in a typical Unix shell, then it's set, because that opens the file in append mode.