Cleanup the public vs private ISCAPI remnants (v9.16)

Since all the libraries are internal now, just cleanup the ISCAPI remnants in isc_socket, isc_task and isc_timer APIs. This means, there's one less layer as following changes have been done:

• struct isc_socket and struct isc_socketmgr have been removed
• struct isc__socket and struct isc__socketmgr have been renamed to struct isc_socket and struct isc_socketmgr
• struct isc_task and struct isc_taskmgr have been removed
• struct isc__task and struct isc__taskmgr have been renamed to struct isc_task and struct isc_taskmgr
• struct isc_timer and struct isc_timermgr have been removed
• struct isc__timer and struct isc__timermgr have been renamed to struct isc_timer and struct isc_timermgr
• All the associated code that dealt with typing isc_ to isc__ and back has been removed.

(cherry picked from commit 16fe0d1f)