s3:util: Move popen wrappers to lib/util

s3:util: Move popen wrappers to lib/util

When linked into Samba3 libraries, sys_popen()/sys_pclose()
cannot be used in lower level libraries because of circular
dependencies.

This patch moves them into common samba-util library.

Extra fix needed to enable easy back-port of code for:

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13964

Signed-off-by: Aliaksei Karaliou <akaraliou@panasas.com>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit 65ea3f2a461c0eeca7d14c4258eea52e19f0ed06)