hooks: Add pre-create hooks for runtime-config manipulation

There's been a lot of discussion over in [1] about how to support the
NVIDIA folks and others who want to be able to create devices
(possibly after having loaded kernel modules) and bind userspace
libraries into the container.  Currently that's happening in the
middle of runc's create-time mount handling before the container
pivots to its new root directory with runc's incorrectly-timed
prestart hook trigger [2].  With this commit, we extend hooks with a
'precreate' stage to allow trusted parties to manipulate the config
JSON before calling the runtime's 'create'.

I'm recycling the existing Hook schema from pkg/hooks for this,
because we'll want Timeout for reliability and When to avoid the
expense of fork/exec when a given hook does not need to make config
changes [3].

[1]: https://github.com/opencontainers/runc/pull/1811
[2]: https://github.com/opencontainers/runc/issues/1710
[3]: https://github.com/containers/libpod/issues/1828#issuecomment-439888059

Signed-off-by: W. Trevor King <wking@tremily.us>

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