Problem:
The default builtin UI client does not declare its client info. This
reduces discoverability and makes it difficult for plugins to identify
the UI.
Solution:
- Call nvim_set_client_info after attaching, as recommended by `:help dev-ui`.
- Also set the "pid" field.
- Also change `ui_active()` to return a count. Not directly relevant to
this commit, but will be useful later.
Problem:
It has long been a convention that references to the builtin terminal UI
should mention "tui", not "term", in order to avoid ambiguity vs the
builtin `:terminal` feature. The final step was to rename term.txt;
let's that step.
Solution:
- rename term.txt => tui.txt
- rename nvim_terminal_emulator.txt => terminal.txt
- `gen_help_html.lua`: generate redirects for renamed pages.
