Adds tests for Readdir and Readdirnames with different n
values. No good way to inject faults during full reads,
though.
Also fixes bug report from fshahriar:
Readdir(0) wasn't behaving like Readdir(-1).
R=rsc, fshahriar
CC=golang-dev
https://golang.org/cl/4529092
Moved the details of how to read a directory
and how to parse the results behind the new
syscall functions ReadDirent and ParseDirent.
Now os needs just one copy of Readdirnames
for the three Unix variants, and it no longer
imports "unsafe".
R=r, r2
CC=golang-dev
https://golang.org/cl/4368048
