jobs: child proc must have a separate process-group

UV_PROCESS_DETACHED compels libuv:uv__process_child_init() to call
setsid() in the child just after fork().  That ensures the process and
its descendants are grouped in a separate session (and process group).

The following jobstart() call correctly groups `sh` and `sleep` in a new
session (and process-group), where `sh` is the "session leader" (and
process-group leader):

    :call jobstart(['sh','-c','sleep 60'])

     SESN  PGRP   PID  PPID  Command
    30383 30383 30383  3620  │  ├─ -bash
    30383 31432 31432 30383  │  │  └─ nvim -u NORC
    30383 31432 31433 30383  │  │     ├─ nvim -u NORC
     8105  8105  8105 31432  │  │     └─ sh -c sleep 60
     8105  8105  8106  8105  │  │        └─ sleep 60

closes #6530
ref: https://stackoverflow.com/q/1046933
ref: https://unix.stackexchange.com/a/404065

Helped-by: Marco Hinz <mh.codebro+github@gmail.com>

Discussion
------------------------------------------------------------------------

On my linux box before this patch, the termclose_spec.lua:'kills job
trapping SIGTERM' test indirectly causes cmake/busted to wait for 60s.
That's because the test spawns a `sleep 60` descendant process which
hangs around even after nvim exits: nvim killed the parent PID, but not
PGID (process-group), so the grandchild "reparented" to init (PID 1).

Session contains processes (and process-groups) which are logically part
of the same "login session". Process-group is a set of
logically/informally-related processes within a session; for example,
shells assign a process group to each "job". Session IDs and PGIDs both
have type pid_t (like PIDs).

These OS-level mechanisms are, as usual, legacy accidents whose purpose
is upheld by convention and folklore.  We can use session-level grouping
(setsid), or we could use process-group-level grouping (setpgid).

Vim uses setsid() if available, otherwise setpgid(0,0).

Windows
------------------------------------------------------------------------

UV_PROCESS_DETACHED on win32 sets CREATE_NEW_PROCESS_GROUP flag.
But uv_kill() does not kill the process-group:
https://github.com/nodejs/node/issues/3617

Ideas:
- Set UV_PROCESS_DETACHED (CREATE_NEW_PROCESS_GROUP), then call
  GenerateConsoleCtrlEvent(CTRL_BREAK_EVENT, pid)
   - Maybe won't work because MSDN says "Only processes that share the
     same console as the calling process receive the signal."
     https://docs.microsoft.com/en-us/windows/console/generateconsolectrlevent
     But CREATE_NEW_PROCESS_GROUP creates a new console ...
     ref https://stackoverflow.com/q/1453520
- Group processes within a "job". libuv does that *globally* for
  non-detached processes: uv__init_global_job_handle.
- Iterate through CreateToolhelp32Snapshot().
   - https://stackoverflow.com/q/1173342
   - Vim does this, see terminate_all()

src/nvim/os/pty_process_unix.c

@@ -145,8 +145,12 @@ void pty_process_teardown(Loop *loop)
   uv_signal_stop(&loop->children_watcher);
 }
 
-static void init_child(PtyProcess *ptyproc) FUNC_ATTR_NONNULL_ALL
+static void init_child(PtyProcess *ptyproc)
+  FUNC_ATTR_NONNULL_ALL
 {
+  // New session/process-group. #6530
+  setsid();
+
   unsetenv("COLUMNS");
   unsetenv("LINES");
   unsetenv("TERMCAP");