Re-land: [mojo-edk] Serialise windows handles into an "extra header" section.

This change introduces an "extra header" section of a channel message
header. On windows, platform handles are place into this section instead
of being appended to the end of a message.

At the same time, make a couple of smaller changes:
1. Allow the number of serialised handles to be less than the number
stated in the constructor.
2. Add a has_mach_ports() helper function.
3. Limit the number of attached handles.

These changes are all prep work for Mach ports support.

This re-land perserves the old wire format and message semantics on ChromeOS
and changes it on all other platforms.

Original change: https://codereview.chromium.org/1740953002

BUG=582468

Review URL: https://codereview.chromium.org/1759033002

Cr-Commit-Position: refs/heads/master@{#379114}

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