Given the other type names "jumps", "vars", etc., the name "buflist"
is somewhat unintuitive.

Since the parameter is already non-primitive, make it an `opts` map
instead of just a list, in case we want to extend it later.

fix #3436

Includes:
vim-patch:8.0.1038: strike-through text not supported

Before this, --embed UIs (without --headless) would not trigger UIEnter.

For TUI, maybe UIEnter isn't useful, but:
- It is less "surprising"/special.
- Makes documentation simpler.
- When TUI becomes a coprocess, it will happen anyway.

"enter"/"leave" is more conventional for Vim events, and
"attach"/"detach" distinction does not gain much.

doc: ginit.vim, gvimrc
fix #3656

Eliminate `expect_err` in favor of `pcall_err` + `eq` or `matches`.

- Rename `meth_pcall`.
- Make `pcall_err` raise an error if the function does not fail.
- Add `vim.pesc()` to treat a string as literal where a Lua pattern is
  expected.