sh: Return status 127 for unknown jobs in wait builtin.

This is required by POSIX, at least for pids that are not known child
processes.

Other problems with job specifications still cause wait to abort with
exit status 2.

PR:		176916
Pull in r183297 from upstream llvm trunk:

  PR15662: Optimized debug info produces out of order function
  parameters

  When a function is inlined we lazily construct the variables
  representing the function's parameters. After that, we add any
  remaining unused parameters.

  If the function doesn't use all the parameters, or uses them out of
  order, then the DWARF would produce them in that order, producing a
  parameter order that doesn't match the source.

  This fix causes us to always keep the arg variables at the start of
  the variable list & in the original order from the source.

Reported by:	avg
MFC after:	1 week
sh(1): Document new features in wait builtin.

PR:		176916
sleep: Explain in a comment why the [EINTR] check is there.

Suggested by:	eadler