[kernel] Return standard exit status on stopped jobs

[ghaerr]

Corrects process exit values returned by waitpid(..., &status, ...) etc to Linux standard. This now aligns with the WIF* macros in <sys/wait.h>.

The standard values for integer value of status are:

Process status       High byte Low Byte
Normal termination   exitvalue 0
Signal termination   0         signo
Stopped              signo     0x7f

sash was updated to properly report stopped jobs and the stopping signal number (either SIGSTOP or SIGTSTP for now).
The Mar 3 1991 version of ash ELKS is using is hopelessly out of date and is fighting hard not to report on Stopped jobs. Comparison with a later May 4 1995 version has tons of code updates in jobs.c when using waitpid(..., WUNTRACED) but unfortunately I've not been able to even get it to compile.

Good shell jobs support in ELKS looks pretty grim, since real support needs the addition of SIGTTIN and SIGTTOU signals, along with the missing tcsetpgrp and tcgetptrp ioctls for working with the controlling terminal.