Standardise license headers and copyright notices

[mato]

This commit unifies the license headers and copyright notices in Solo5
and ukvm source files. Original source files use one of the following
standard headers:

For the Solo5 unikernel:

Copyright (c) 2015-2017 Contributors as noted in the AUTHORS file

This file is part of Solo5, a unikernel base layer.

For ukvm:

Copyright (c) 2015-2017 Contributors as noted in the AUTHORS file

This file is part of ukvm, a unikernel monitor.

The header is followed by the standard ISC license text.

Where a source file contains existing third-party licenses or copyright
notices these have been preserved and the Solo5 header has been added to
the top of the file.

Further, this commit updates the copyright notice in the top-level
LICENSE file to read:

Copyright (c) 2015-2017 Contributors as noted in the AUTHORS file

and adds a new AUTHORS file containing:

1. copyright notices for corporate contributors,
2. a list of individual contributor's names and their affiliation(s),
   if any.

I have populated the AUTHORS file with all contributors to date based on
the information in the git history.

[avsm]

This lgtm, with the refinement of adding any corporate affiliations after contributor's names in the "Individual Contributors" section above. My reasoning is that the copyright ownership rests with the corporate contributor in this case, and so it provides a mapping from committer to copyright owner in that case, but still leaves a suitable place for individual contributors to add their name too.

[mato]

Thanks @avsm, this is a good idea. I've addressed it and re-pushed this as 07ab01f.