fix #8428

- 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.

This is better practice in general, and allows to remove the "helptags
ALL" hacks.

Ref: https://github.com/neovim/neovim/issues/8824
Ref: https://github.com/neovim/neovim/commit/f1b67c3453c

* Makefile: fix dependencies with regard to helptags

- use the file as the main target to avoid unnecessary triggering
- use "make oldtest" on Travis to ensure it gets built

This extra retry() was removed (at my suggestion) in 5b94a2977a22, but
it is probably needed: jobwait(…, timeout=0) could return while
channel_process_exit_cb() is still queued (so TermClose event didn't
fire yet).

    20:46:21,288 INFO  - not ok 547 - TermClose event triggers when fast-exiting terminal job stops
    20:46:21,288 INFO  - # test/functional/autocmd/termclose_spec.lua @ 20
    20:46:21,288 INFO  - # Failure message: ./test/functional/helpers.lua:98: Vim:E121: Undefined variable: g:test_termclose
    20:46:21,288 INFO  - # stack traceback:
    20:46:21,288 INFO  - # 	./test/functional/helpers.lua:98: in function 'eval'
    20:46:21,288 INFO  - # 	test/functional/autocmd/termclose_spec.lua:25: in function <test/functional/autocmd/termclose_spec.lua:20>

The message is useless, it doesn't even mention the event name.

vim_dev discussion:
https://groups.google.com/forum/#!msg/vim_dev/RTbq58TMq9w/Xr4rSoUTCgAJ

ref: https://github.com/vim/vim/issues/4300

Problem: delete-with-register dispatches TextYankPost before updating yank registers
Solution: Add flag to op_yank().

Fixes #10225

Fixes https://github.com/neovim/neovim/issues/3757.

    [  ERROR   ]...neovim/neovim/test/functional/autocmd/termclose_spec.lua @ TermClose event triggers when fast-exiting terminal job stops
    test/functional/helpers.lua:96: Vim(call):E900: Invalid channel id

    stack traceback:
            test/functional/helpers.lua:96: in function 'request'
            test/functional/helpers.lua:254: in function 'command'
            ...neovim/neovim/test/functional/autocmd/termclose_spec.lua:23: in function <...neovim/neovim/test/functional/autocmd/termclose_spec.lua:20>

Unfortunately we cannot indiscriminately replace xfree() with
XFREE_CLEAR(), because comparing pointers after freeing them is a common
pattern. Example in `tv_list_remove_items()`:

    xfree(li);
    if (li == item2) {
      break;
    }

Instead we can do it selectively/explicitly.

ref #1375