selftest: Add linux namespace support (USE_NAMESPACES=1)

selftest: Add linux namespace support (USE_NAMESPACES=1)

This hooks up the selftest/ns/* scripts added earlier with the selftest
system, so developers can optionally run a testenv or test using linux
namespaces instead of socket-wrapper.

The idea is this is experimental functionality that we can extend
further in future, in order to make testing Samba more versatile.

+ The top-level WAF script now does an 'unshare' to create a new
top-level 'selftest' namespace in which to create the testenv(s).
+ selftest.pl creates a common 'selftest0' bridge to connect together
the individual DCs.
+ Update Samba.pm so it can use real IPs instead of loopback addresses.
In fork_and_exec(), we add a couple of hooks so that the binary gets
started in a different namespace (using unshare/start_in_ns.sh), and
the parent process connects the new child namespace up to the common
selftest0 bridge (using add_bridge_iface.sh).

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>