std.net.curl: Implement seeking in basic HTTP requests

[CyberShadow]

This enables reposting the data after a redirect, for example.

[andralex]

Any documentation needs?

[braddr]

also.. unittests.

[CyberShadow]

Any documentation needs?

It's strictly an implementation improvement (which I would expect to work without it being specified in the documentation).

also.. unittests.

For unittests, an appropriately-configured web server is needed. However, I've asked Jonas to publish the source code to his test service, so we could submit patches to allow new unittests. The source has been made available after I submitted this pull request, so I'll add an unit test now.

[CyberShadow]

Done. Requires jcd/d-lang-testservice#1 when PHOBOS_TEST_ALLOW_NET is set in the environment.

[CyberShadow]

The change has been pushed to the test server, the network unittests pass, so this should be good to merge now.

[alexrp]

LGTM, but let's wait for the auto tester to go through it.

[alexrp]

Seems to pass, so merging.