ash: additional support for background jobs

The rationale for the jobs builtin in POSIX notes:

   The jobs utility is not dependent on the job control option,
   as are the seemingly related bg and fg utilities because jobs
   is useful for examining background jobs, regardless of the
   condition of job control. When the user has invoked a set +m
   command and job control has been turned off, jobs can still be
   used to examine the background jobs associated with that current
   session. Similarly, kill can then be used to kill background
   jobs with kill %<background job number>.

Although ash in busybox-w32 doesn't support job control it can
handle background jobs.

Allow the ASH_JOB_CONTROL setting to enable certain features:

- the jobs builtin;
- killing of jobs by job id in the kill builtin;
- monitoring of changes to jobs.

Since process groups aren't supported it's necessary to kill the
processes constituting a background job individually.  When doing
this we ask kill(2) to kill all children too.

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