namespaces: cleanup the code managed with PID_NS option

Just like with the user namespaces, move the namespace management code into
the separate .c file and mark the (already existing) PID_NS option as "depend
on NAMESPACES"

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Cc: Kirill Korotaev <dev@sw.ru>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

init/Kconfig

@@ -214,18 +214,6 @@ config TASK_IO_ACCOUNTING
 
 	  Say N if unsure.
 
-config PID_NS
-	bool "PID Namespaces (EXPERIMENTAL)"
-	default n
-	depends on EXPERIMENTAL
-	help
-	  Suport process id namespaces.  This allows having multiple
-	  process with the same pid as long as they are in different
-	  pid namespaces.  This is a building block of containers.
-
-	  Unless you want to work with an experimental feature
-	  say N here.
-
 config AUDIT
 	bool "Auditing support"
 	depends on NET
@@ -442,6 +430,18 @@ config USER_NS
 	  to provide different user info for different servers.
 	  If unsure, say N.
 
+config PID_NS
+	bool "PID Namespaces (EXPERIMENTAL)"
+	default n
+	depends on NAMESPACES && EXPERIMENTAL
+	help
+	  Suport process id namespaces.  This allows having multiple
+	  process with the same pid as long as they are in different
+	  pid namespaces.  This is a building block of containers.
+
+	  Unless you want to work with an experimental feature
+	  say N here.
+
 config BLK_DEV_INITRD
 	bool "Initial RAM filesystem and RAM disk (initramfs/initrd) support"
 	depends on BROKEN || !FRV