os: avoid second fcntl syscall in NewFile on unix

CL 494915 introduced an additional fcntl(F_GETFL) syscall to determine
whether the file is in append-only mode. The existing unix.IsNonblock
call also issues an fcntl(F_GETFL) syscall. The two can be combined and
both the append-only mode and the non-blocking flags can be determined
from that syscall's result.

Change-Id: I915589ed94e079f6abaa2fd0032ef01f78698f7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/497075


Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>