It was replaced by the "child queue" concept (MultiQueue).

Introduce multiqueue_process_priority() to process only events at or
above a certain priority.

Asynchronous API functions are served immediately, which means pending
input could change the state of Nvim shortly after an async API function
result is returned.

nvim_get_mode() is different:
  - If RPCs are known to be blocked, it responds immediately (without
    flushing the input/event queue)
  - else it is handled just-in-time before waiting for input, after
    pending input was processed. This makes the result more reliable
    (but not perfect).

Internally this is handled as a special case, but _semantically_ nothing
has changed: API users never know when input flushes, so this internal
special-case doesn't violate that. As far as API users are concerned,
nvim_get_mode() is just another asynchronous API function.

In all cases nvim_get_mode() never blocks for more than the time it
takes to flush the input/event queue (~µs).

Note: This doesn't address #6166; nvim_get_mode() will provoke #6166 if
e.g. `d` is operator-pending.

Closes #6159

Disable CommandLineToArgvW-standard quoting for cmd.exe.

libuv assumes spawned processes follow the convention expected by
CommandLineToArgvW(). But cmd.exe is non-conformant, so for cmd.exe:
- With system([]), the caller has full control (and responsibility) to
  quote arguments correctly.
- With system(''), shell* options are used.

libuv quoting is disabled if argv[0] is:
- cmd.exe
- cmd
- $COMSPEC resolving to a path with filename cmd.exe

Closes #6329
References #6387

Also fixed an error in path_fnamecmp().

Also fixes buffer reusage in setmatches() and complete().