Don't panic, crash, or return references to uninitialized memory when
ParseDirent is passed invalid input.

Move common dirent parsing to syscall.go with minimal platform-specific
functions in syscall_$GOOS.go.

Fixes #15653

Change-Id: I5602475e02321fe381064488401c14b33bec6886
Reviewed-on: https://go-review.googlesource.com/23780
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>