helix-term: send the STOP signal to all processes in the process group (#3546)

* helix-term: send the STOP signal to all processes in the process group

From kill(3p):

    If pid is 0, sig shall be sent to all processes (excluding an unspecified set
    of  system processes) whose process group ID is equal to the process group ID
    of the sender, and for which the process has permission to send a signal.

This fixes the issue of running `git commit`, attempting to suspend
helix with ^Z, and then not regaining control over the terminal and
having to press ^Z again.

* helix-term: use libc directly to send STOP signal

* helix-term: document safety of libc::kill

* helix-term: properly handle libc::kill's failure

I misread the manpage for POSIX `kill` -- it returns `-1` in
the failure case, and sets `errno`, which is retrieved via
`std::io::Error::last_os_error()`, has its string representation printed
out, and then exits with the matching status code (or 1 if, for whatever
reason, there is no matching status code).

* helix-term: expand upon why we need to SIGSTOP the entire process group

Also add a link back to one of the upstream issues.

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