Dissociate controlling tty better on Darwin

Dissociate controlling tty better on Darwin

* src/process.c (dissociate_controlling_tty): New function.
(create_process): Use it to dissociate controlling tty if setsid
fails, which happens on Darwin after a vfork (Bug#33154).
Do this on all platforms, not just on Darwin, as a similar
problem is plausible elsewhere.
* src/callproc.c (call_process): Use the new function here, too,
for consistency and to avoid duplicate code.