Show useful errors when posix_spawn() doesn't do so

Show useful errors when posix_spawn() doesn't do so

The posix_spawn() function may not detect that the command to run is
invalid when it's invoked.  Instead, it will run then exit with
error code 127.  If that happens do our best to present the user
with a useful error message.

* src/job.h (struct child): Add cmd_name to hold the command we ran.
* src/job.c (start_job_command): On success, remember the cmd_name.
(reap_children): On exit 127, stat cmd_name and show a useful error.
(free_child): Free cmd_name.