math/rand: auto-seed global source

Implement proposal #54880, to automatically seed the global source.

The justification for this not being a breaking change is that any
use of the global source in a package's init function or exported API
clearly must be valid - that is, if a package changes how much
randomness it consumes at init time or in an exported API, that
clearly isn't the kind of breaking change that requires issuing a v2
of that package. That kind of per-package change in the position
of the global source is indistinguishable from seeding the global
source differently. So if the per-package change is valid, so is auto-seeding.

And then, of course, auto-seeding means that packages will be
far less likely to depend on the specific results of the global source
and therefore not break when those kinds of per-package changes
happen in the future.

Seed(1) can be called in programs that need the old sequence from
the global source and want to restore the old behavior.
Of course, those programs will still be broken by the per-package
changes just described, and it would be better for them to allocate
local sources rather than continue to use the global one.

Fixes #54880.

Change-Id: Ib9dc3307b97f7a45587a9cc50d81f919d3edc7ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/443058
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>

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