Co-authored-by: Dustin S. <dstackmasta27@gmail.com>
Co-authored-by: Ferenc Fejes <fejes@inf.elte.hu>
Co-authored-by: Maria José Solano <majosolano99@gmail.com>
Co-authored-by: Yochem van Rosmalen <git@yochem.nl>
Co-authored-by: brianhuster <phambinhanctb2004@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Also print stderr error messages with ErrorMsg highlight group.

Result of `make iwyu` (after some "fixups").

loop->children might have been a linked list because used to be
modified in place while looped over. However the loops that exists
rather schedules events to be processed later, outside of the loop,
so this can not happen anymore.

When a linked list is otherwise useful it is better to use
lib/queue_defs.h which defines an _intrusive_ linked list (i e it
doesn't need to do allocations for list items like klist ).

Problem:
- "process" is often used as a verb (`multiqueue_process_events`), which
  is ambiguous for cases where it's used as a topic.
- The documented naming convention for processes is "proc".
  - `:help dev-name-common`
- Shorter is better, when it doesn't harm readability or
  discoverability.

Solution:
Rename "process" => "proc" in all C symbols and module names.

Problem:
The name `os_inchar` (from Vim's old `mch_inchar`) is ambiguous:
"inchar" sounds like it could be reading or enqueuing (setting) input.
Its docstring is also ambiguous.

Solution:
- Rename `os_inchar` to `input_get`.
- Write some mf'ing docstrings.
- Add assert() in TRY_READ().