Make lib/isc/app.c opaque and thread-safe

Make lib/isc/app.c opaque and thread-safe

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}