fix regression introduced by recent switch to nih_locals

1. do_collect_tasks was still doing strncat(path, x, MAXPATHLEN-1)
   but path was not pre-allocated to MAXPATHLEN
2. collect_tasks needs to pass a nih_alloc'ed path just as do_collect_tasks
   does
3. pass pointer to the nih_alloc'd path so we can append to it.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>