unshare: add --fork options for pid namespaces

unshare: add --fork options for pid namespaces

The ability of unshare to launch a new pid namespace is a bit limited.
The first process in the namespace is expected to be the "init" for it.
When it's not, you get bad behavior.

For example, trying to launch a shell in a new pid namespace fails very
quickly:
	$ sudo unshare -p dash
	# uname -r
	3.8.3
	# uname -m
	dash: 2: Cannot fork
	# ls -ld /
	dash: 3: Cannot fork
	# echo $$
	1324

For this to work smoothly, we need an init process to actively watch over
things.  But forcing people to re-use an existing init or write their own
mini init is a bit overkill.  So let's add a --fork option to unshare to
do this common bit of book keeping.  Now we can do:
	$ sudo unshare -p --fork dash
	# uname -r
	3.8.3
	# uname -m
	x86_64
	# ls -ld /
	drwxr-xr-x 22 root root 4096 May  4 14:01 /
	# echo $$
	1

Thanks to Michael Kerrisk for his namespace articles on lwn.net

[kzak@redhat.com: - fix "forkif logic, remove --mount-proc]

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Karel Zak <kzak@redhat.com>