Consolidate reactor backends into parameterized templates

The epoll, kqueue, and select backends each had 13 nearly-identical
per-protocol files for the socket, service, and acceptor types of
TCP, UDP, and Unix stream/datagram. Replace those 39 files with
shared parameterized templates plus per-backend traits and types
files, dropping each backend from 14 files (the 13 plus the
scheduler) to 3 (scheduler, traits, types).

New per-backend files:
- {backend}_traits.hpp: platform-specific behavior (socket creation
  flags, write and accept policies, fd configuration, stream socket
  hook for SO_LINGER, descriptor_state)
- {backend}_types.hpp: named final classes that instantiate the
  shared templates

New shared templates in detail/reactor/:
- reactor_stream_ops.hpp and reactor_datagram_ops.hpp: parameterized
  op types
- reactor_socket_finals.hpp: reactor_stream_socket_impl,
  reactor_dgram_socket_impl, reactor_acceptor_impl
- reactor_service_finals.hpp: per-protocol service implementations
  plus do_open_socket, do_assign_fd, and do_open_acceptor helpers
- reactor_backend.hpp: the accept() method body, which needs all
  the named types to be complete

The existing reactor base templates (reactor_basic_socket,
reactor_stream_socket, reactor_datagram_socket, reactor_acceptor)
gain ImplBase and Endpoint template parameters so a single
template works for both IP and Unix domain sockets.

io_context.cpp and the native/ tcp_socket, tcp_acceptor, and
udp_socket headers switch to including {backend}_types.hpp in
place of the individual service headers.

Author: mvandeberg