* fixed hang issue with --headless and -r option specified

Calling the do_more_prompt function in headless mode will freeze neovim because it is eventally in the input-accepting state (the same as waiting for --more--).

* fixed "Press ENTER or type command to continue" to be suppressed

If in headless mode, we need to exit at this point. If we continue, we will enter the normal mode and the message "Press ENTER or type command to continue" will be displayed and we will be in the input waiting state.

* fixed functional ex_cmds tests

* Revert "fixed "Press ENTER or type command to continue" to be suppressed"

This reverts commit a02dc40e3b3fad69cedcde6abe1bd4efe39ab102.

* Revert "fixed functional ex_cmds tests"

This reverts commit 3bdb8da20acf34673b2c2028d15e7ce6da4c792a.

* fixed conditional again

* added test for fixed hang issue with --headless (#11386)

(cherry picked from commit c6dc397801d0566b583d4916220a60f17e609e25)

ref 77594d55f709

Closes https://github.com/neovim/neovim/issues/10031

* 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

Problem:    When starting with multiple tabs file messages is confusing.
Solution:   Set 'shortmess' when loading the other tabs. (Christian Brabandt)
https://github.com/vim/vim/commit/c75e81262347e47a69faabd72caf89fec3f06e8f

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

- Move os_name() up to "global helpers".
- Rename it to is_os().
- Make it depend on uname() instead of a running Nvim instance.

- Update the test to work with changes since it was originally written.
- Keep the test pending() because it still fails:

      Expected objects to be the same.
      Passed in:
      (table: 0x50ce9e38) {
        [1] = {
          [1] = 'notification'
          [2] = 'stderr'
          [3] = {
            [1] = 0
            [2] = {
              [1] = '' } } }
       *[2] = {
          [1] = 'notification'
          [2] = 'stdout'
         *[3] = {
            [1] = 0
           *[2] = {
             *[1] = '' } } }
        [3] = {
          [1] = 'notification'
          [2] = 'exit'
          [3] = {
            [1] = 0
            [2] = 143 } } }
      Expected:
      (table: 0x50ce9870) {
        [1] = {
          [1] = 'notification'
          [2] = 'stderr'
          [3] = {
            [1] = 0
            [2] = {
              [1] = '' } } }
       *[2] = {
          [1] = 'notification'
          [2] = 'stdout'
         *[3] = {
            [1] = 0
           *[2] = {
             *[1] = 'abcdef' } } }
        [3] = {
          [1] = 'notification'
          [2] = 'stdout'
          [3] = {
            [1] = 0
            [2] = {
              [1] = '' } } } }

Problem:  If a job exits while waiting on another job, the on_exit
          handler is queued but f_jobwait() skips it.
Solution: Always do process_wait(), so that handlers are run during
          f_jobwait().

fix #8302

Test case:
    $ BUSTED_ARGS="--repeat=2000 --no-keep-going" TEST_FILE=test/functional/core/job_spec.lua TEST_FILTER=waiting make functionaltest

Failure example (macOS CI):
    FAILED  test/functional/core/job_spec.lua: jobs jobwait will run callbacks while waiting
    test/functional/core/job_spec.lua:606: Expected objects to be the same.
    Passed in:
    (table: 0x1be77c80) {
      [1] = 'notification'
      [2] = 'wait'
     *[3] = {
       *[1] = 3 } }
    Expected:
    (table: 0x1be77d10) {
      [1] = 'notification'
      [2] = 'wait'
     *[3] = {
       *[1] = 4 } }
    stack traceback:
      test/functional/core/job_spec.lua:606: in function <test/functional/core/job_spec.lua:583