runtime: drop CLONE_PARENT
runtime: drop CLONE_PARENT

The functionality we want (shared ppid) is implied
by CLONE_THREAD already, and CLONE_PARENT
causes problems if the Go program is pid 1 (init).

See issue 1406 for more details.

Fixes issue 1406.

R=adg, iant
CC=golang-dev
http://codereview.appspot.com/3971044

tv42/old-go,ssrl/go,gnanderson/go,ssrl/go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,ssrl/go,tv42/old-go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,ssrl/go,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,ssrl/go,tv42/old-go,tv42/old-go,gnanderson/go,dsriram/golang,apparentlymart/golang,ssrl/go,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang