test: Add nsholder utility

In our test scripts we need to do some ugly parsing of /proc and/or pstree
output in order to get the PIDs of processes running in namespaces so that
we can connect to those namespaces with nsenter or pasta.

This is actually a pretty tricky problem with standard tools.  To determine
the PID from the outside of the namespace we need to know how the process
of interest is related to the unshare or pasta process (child? one of
several children? grandchild?) as well as then parsing /proc or ps output.
This is slightly awkward now, and will get worse with future changes I'd
like to make to have processes are dispatched.

The obvious solution would be to have the process of interest (which we
control) report its own PID, but that doesn't work easily, because it is in
a PID namepace and sees only its local PID not the global PID we need to
address it from outside.

To handle this, add a small custom tool, "nsholder".  This takes a path
and a mode parameter.  In "hold" mode it will create a unix domain socket
bound to the path and listening.  In "pid" mode it will get the "hold"ing
process's pid via the unix socket using SO_PEERCRED, which translates
between PID namespaces.  In "stop" mode it will send a message to the
socket causing the "hold"ing process to clean up and exit.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

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