Check all child processes for exit in SIGCHLD handler

channel.c: refactor spaghetti code

Without this patch the following commands would crash:
    :call rpcstart('unknown_command')

    :set sh=unknown_command
    :call jobstart('ls',{'rpc':1})

If a second and third child exit while we are already in the handler, we
will only see a single additional SIGCHLD.  Therefore the handler must
not stop after processing a single child but should check all children.

Fixes #8740

channel.c: WIP remove redundant method check and added FUNC_ATTR_NONNULL_ALL macro

channel.c channel_defs.h helpers.c: added Error field to RequestEvent, added no_op handler func

channel.c: use const char* instead of string and cleanup

channel.c; channel_defs.h; helpers.c: removed error from event again; send errors directly to the channel without using handlers and events

channel.c: fixed memory leak and lint errors

api/private/dispatch.c; api/vim.c; msgpack_rpc/channel.c msgpack_rpc/helpers.c added Error* field to msgpack_get_handler_for; further refactored channel.c

channel.c:323 changed order of evaluation in if statement

channel.c: removed superflous whitespace

dispatch.c: review comment

Only waitpid() for processes that we care about

It seems as though in an AppImage there's an extra child process that
dies at some early point, before we have set up a SIGCHLD handler.  So
when we later get a SIGCHLD from a child that we do care about,
waitpid(-1, ...) tells us about the extra child - and we don't notice
that the interesting child has exited.

Or something like that!

See also:

* https://patchwork.kernel.org/patch/9949491/ in which perf hit
something similar
* discussion at the AppImage repository:
https://github.com/AppImage/AppImageKit/issues/812#issuecomment-404662110.

Fix is to be explicit about which process we are waitpid()'ing for, so
we never need be distracted by children that we don't know about.

Fixes #8104

Problem:    The :global command does not work recursively, which makes it
            difficult to execute a command on a line where one pattern matches
            and another does not match. (Miles Cranmer)
Solution:   Allow for recursion if it is for only one line. (closes vim/vim#1760)
https://github.com/vim/vim/commit/f84b122a99da75741ae686fabb6f81b8b4755998