devel/libuv: Update 1.38.1 -> 1.39.0

Release notes at https://github.com/libuv/libuv/releases/tag/v1.39.0.

While here, take maintainership after an offer by thierry@, since this
is an important dependency for the Node.js ports.

Approved by: thierry (maintainer)
Sponsored by: Miles AS