ProcessUNIX: close /proc entry before starting recursion

If a group of processes needs to be terminated recursively the entry in /proc
doesn't need to be kept open during the recursion as all the interesting
information was already read from it. Especially in case of deeper recursion
this lowers the memory pressure.

Change-Id: I95b969f42c8dd68b57cdaf8fc1b705f358f1dc58