procdesc: respect __WALL flag even for procdesc

If the __WALL flag is set for a wait4 operation, respect it
and report any children even if they have extant process
descriptors.

(The __WALL flag is used internally in the kernel when the
init-like task of a PID namespace terminates, and takes down
all other tasks in that namespace with it.  This should occur
even if there are process descriptors.)
procdesc: fix anon_new_inode_getfile comment