util/syspolicy/policyclient: add policyclient.Client interface, start plumbing

This is step 2 of ~4, breaking up #14720 into reviewable chunks, with
the aim to make syspolicy be a build-time configurable feature.

Step 1 was #16984.

In this second step, the util/syspolicy/policyclient package is added
with the policyclient.Client interface.  This is the interface that's
always present (regardless of build tags), and is what code around the
tree uses to ask syspolicy/MDM questions.

There are two implementations of policyclient.Client for now:

1) NoPolicyClient, which only returns default values.
2) the unexported, temporary 'globalSyspolicy', which is implemented
   in terms of the global functions we wish to later eliminate.

This then starts to plumb around the policyclient.Client to most callers.

Future changes will plumb it more. When the last of the global func
callers are gone, then we can unexport the global functions and make a
proper policyclient.Client type and constructor in the syspolicy
package, removing the globalSyspolicy impl out of tsd.

The final change will sprinkle build tags in a few more places and
lock it in with dependency tests to make sure the dependencies don't
later creep back in.

Updates #16998
Updates #12614

Change-Id: Ib2c93d15c15c1f2b981464099177cd492d50391c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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