libvterm fork

Important: This is currently not used by neovim. Vterm has instead been
bundled into the neovim repo itself:
https://github.com/neovim/neovim/tree/master/src/nvim/vterm. All pull requests
should be sent to neovim instead.

This is a fork of https://www.leonerd.org.uk/code/libvterm. An exact mirror
of upstream is in the branch mirror.