syscall: add Cloneflags to Linux SysProcAttr.

Also use clone(2) syscall instead of fork().

Fixes issue 6214.

R=golang-dev, bradfitz, dave
CC=golang-dev
https://codereview.appspot.com/13159044
os/user: Current support on Plan 9

Current for Plan 9 is implemented with /dev/user for
Uid/Gid/Username/Name, and $home environment variable for
HomeDir.

Implementing Lookup/LookupId is not done, which would
require parsing /adm/users. It is unclear of how much benefit
this would be.

R=golang-dev
CC=bradfitz, golang-dev, r
https://codereview.appspot.com/13203043

Committer: Brad Fitzpatrick <bradfitz@golang.org>