Use typedef DBusSocket for sockets fd's to avoid conversion warnings.

Use typedef DBusSocket for sockets fd's to avoid conversion warnings.

[smcv: remove unneeded and invalid dbus-sysdeps.h from public header;
make prototype of _dbus_socketpair() consistent; undo conversion
of getaddrinfo result from int to SOCKET; don't call
_dbus_return_val_if_fail() from internal function]

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=89444