When a task manager is created, we can now specify an `isc_nm`
object to associate with it; thereafter when the task manager is
placed into exclusive mode, the network manager will be paused.
This work cleans up the API which includes couple of things:
1. Make the isc_appctx_t type fully opaque
2. Protect all access to the isc_app_t members via stdatomics
3. sigwait() is part of POSIX.1, remove dead non-sigwait code
4. Remove unused code: isc_appctx_set{taskmgr,sockmgr,timermgr}
- some of these tests are obsolete and should be cleared up,
others overlap with ATF tests and may be removed later.
for now, let's just tidy up the bin/tests directory by
moving these files down a level.
