os, syscall: refactor Unix directory parsing

os, syscall: refactor Unix directory parsing

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