user namespaces: introduction

Here is a small patchset I've been sitting on for awhile
to make signaling mostly subject to user namespaces.  In
particular,

	1. store user_namespace in user struct
	2. introduce CAP_NS_OVERRIDE
	3. require CAP_NS_OVERRIDE to signal another user namespace

The first step should have been done all along.  Else wouldn't
a hash collision on (ns1, uid) and (ns2, uid), however unlikely,
give us wrong results at uid_hash_find()?

The main remaining signaling+userns issue is of course the
siginfo.  Tacking a userns onto siginfo is a pain due to
lifetime mgmt issues.  I haven't decided whether to just
catch all the callers and fake uid=0 if user namespaces
aren't the same, introduce some unique non-refcounted id to
represent (user,user_ns), or find some other way to deal with
it.

thanks,
-serge
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