[kernel,cmds] Revise waitpid for stopped jobs, enhance ash, sash & kill signal handling

[ghaerr]

Revised kernel handling of waitpid for stopped processes; now returns the stopped status just once, per spec. Note that the WUNTRACED flag has to be passed in order to get status of any stopped processes. This simplifies the kernel implementation a bit, since the kernel will never wait for a process to restart when WUNTRACED is passed, regardless of the WNOHANG flag.

The above fix allowed ash to be made to work with its builtin jobs command, which is now useful for displaying all background processes (both asynchronous commands started with '&' as well as any stopped commands). Note that actual "jobs" support is not implemented, so fg is not present. Nonetheless, the jobs enhancement allows a quicker way of finding background jobs without all the details of ps.

The kill command and the sash kill builtin now support many more signal names (rather than signal numbers) to be specified in the command line. This now allows using kill -CONT pid to continue a stopped process, or kill -STOP pid to stop one, for instance. The kill man page is also updated for full details.

Here's an example of running a program a.out which displays X's until stopped by ^Z or interrupted by ^C, and using jobs for displaying processes running under the shell:

Note that stopping jobs that are waiting for input on the same terminal as the shell doesn't work well, and likely won't ever, due to the complications of implementing full job control in both the shell and kernel.