Commit 3991faf464 moved search into the registry
package, which also made the `dockerversion` package a dependency for registry,
which brings additional (indirect) dependencies, such as `pkg/parsers/kernel`,
and `golang.org/x/sys/windows/registry`.
Client code, such as used in docker/cli may depend on the `registry` package,
but should not depend on those additional dependencies.
This patch moves setting the userAgent to the API router, and instead of
passing it as a separate argument, includes it into the "headers".
As these headers now not only contain the `X-Meta-...` headers, the variables
were renamed accordingly.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
cb76638699