make syscall use strings for file names

make syscall use strings for file names
tweak os to adjust
move StringToBytes into syscall, at least for now

this program still works:

	package main

	import os "os"

	func main() {
		os.Stdout.WriteString("hello, world\n");
		a, b := os.NewFD(77).WriteString("no way");
		os.Stdout.WriteString(b.String() + "\n");
	}

R=rsc
DELTA=263  (59 added, 176 deleted, 28 changed)
OCL=15153
CL=15153