Fix race condition in wfShellExec()

Especially when executing commands that return a relatively lot of data in stdout
quickly, proc_get_status() may return that command has terminated before everything
has been read from pipes. Handle this case by continuing to perform
non-blocking select on the process's streams until all remaining data has been
read.

Bug: 67870
Change-Id: I050292dbb76821f66a15f937bf3aaf4defe67687