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

* tests: move os_kill to functional helpers

* tests: fix system_spec when run with clipboard manager

Replaces "xclip" with a dedicated helper program.

Fixes: https://github.com/neovim/neovim/issues/4900#issuecomment-501866842

This avoids getting "stuck".  If user actually _wants_ to get stuck
forever, they could use `:sleep` or specify a really big `interval`.

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.

- Always load files when cleaning up jumplist.
  - For Shada: avoids writing duplicate entries, which happens when you read
    from a shada file with duplicate entries (merging the jumplist while
    writing sometimes produces duplicate entries, bug?) and then write right
    away (i.e.: without any `:jumps`, `getjumplist()`, or any jump movement,
    that is: nothing that calls `cleanup_jumplist` with `loadfiles == true`).
  - For Context: avoids non-idempotent behavior for the same reason (i.e.:
    first call to `shada_encode_jumps` does not remove duplicate entries).

- Do not set pcmark when dumping jumplist for Context.
  - Retrieving current Context shouldn't add an entry to the jumplist
    (which will be removed by a subsequent `cleanup_jumplist` anyway, i.e.:
    tail entry matching current position), just act like `getjumplist` for
    instance.

ref #10768

NB: the `!(flags & SOPT_GLOBAL)` exception is for 'statusline'.
Because `:set statusline=...` sets the global value for _all_ windows,
`:setlocal` is the best we can do there. This is a one-of-a-kind option
that doesn't work like any other option.