syscall: add a way (through SysProcAttr?) to use Linux clone(2) flags in fork/exec.

[remyoudompheng]

For systems programming (container-like) I would like to access for example through
syscall.SysProcAttr clone flags that control the properties of forked child processes
(e.g. CLONE_NEWPID, CLONE_NEWNS, CLONE_NEWNET, CLONE_NEWUTS).

I have prepared 2 draft CLs:
- add CLONE_* constants to syscall: https://golang.org/cl/13160043/
- use clone() instead of fork() in syscall/exec_linux.c:
https://golang.org/cl/13159044/

[remyoudompheng]

Comment 1:

I forgot to mention this is for Linux.

[alberts]

Comment 2:

Mount namespaces are awesome. Being able to call Unshare early in a Go program would be
nice.

[remyoudompheng]

Comment 3:

This issue was updated by revision d4f719e.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13160043

[remyoudompheng]

Comment 4:

This issue was closed by revision 534c67a.

Status changed to Fixed.