pgrep: Do not memleak the contents of proc_t.

pgrep: Do not memleak the contents of proc_t.

memset()ing task and subtask inside their loops prevents free_acquired()
(in readproc() and readtask()) from free()ing their contents (especially
cmdline and environ).

Our solution is not perfect, because we still memleak the very last
cmdline/environ, but select_procs() is called only once, so this is not
as bad as it sounds.

It would be better to leave subtask in its block and call
free_acquired() after the loop, but this function is static (not
exported).

The only other solution is to use freeproc(), but this means replacing
the stack task/subtask with xcalloc()s, thus changing a lot of code in
pgrep.c (to pointer accesses).

Hence this imperfect solution for now.