worker task: allow to configure path and owner/group

And application now needs to call init_worker_tasks() before using
worker tasks.

Notable changes:
- need to call  init_worker_tasks() before using worker tasks.
- create_task_log_dirs() ís called inside init_worker_tasks()
- removed UpidExt trait
- use atomic_open_or_create_file()
- remove pbs_config and pbs_buildcfg dependency

src/server/mod.rs

@@ -46,9 +46,6 @@ pub fn our_ctrl_sock() -> String {
     ctrl_sock_from_pid(*PID)
 }
 
-mod upid;
-pub use upid::*;
-
 mod worker_task;
 pub use worker_task::*;