socket: bind connect and bind minimally.

Close #158.
* specs/posix_spec.yaml (accept, bind, connect, getaddrinfo)
(listen, recv, recvfrom, send, sendto, setsockopt, shutdown)
(socket, socketpair): Move from here...
* specs/posix_sys_socket_spec.yaml (accept, bind, connect)
(getaddrinfo, listen, recv, recvfrom, send, sendto, setsockopt)
(shutdown, socket, socketpair): New file. ...to here.
(bind, connect): Adjust specifications to expect 0 returned upon
success.
* specs/posix_deprecated_spec.yaml (bind, connect): New file.
Check for backward compatible return types when imported from
posix module.
* specs/specs.mk (specl_SPECS): Add
specs/posix_sys_socket_spec.yaml and
specs/posix_deprecated_spec.yaml.
* ext/posix/sys/socket.c (bind, connect): Simplify accordingly.
* examples/socket.lua: Adjust to new style submodule imports.
* lib/posix/deprecated.lua (bind, connect): Write wrapper
functions to implement `true` return API.
* local.mk (dist_luaposix_DATA): Add lib/posix/deprecated.lua.
* lib/posix.lua.in: Import deprecated APIs.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>