netservices: remove all old conditional code

Change-Id: Idf3773c081ce103647c5dbea6e82c22bfc5606c9

libbnetapi: BUrlRequest now outputs to BDataIO

Previously, BUrlRequest returns data received via a callback that can't
return any value. This approach have several issues:

- It's not possible to signify failures to the request.
- Users have to implement custom listeners just to handle the common
case of outputting to a buffer/file/etc.
- The received data has to be serialized into BMessage when
BUrlProtocolDispatchingListener is employed. This can cause a
noticible slowdown in real-world scenarios as evident by #10748.

With this change, BUrlRequest will output directly into a BDataIO, which
exposes a richer API for request handlers to work with (for example a
BitTorrent client can request a BPositionIO for non-linear data
delivery), as well as simplifying common cases for users.

The adaptation only requires one additional API:
BHttpRequest::SetStopOnError(). This API simply instructs the HTTP
request handler to cancel the request if an HTTP error is occurred.

Change-Id: I4160884d77bff0e7678e0a623e2587987704443a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3084
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

libnetservices.a: Put the experimental API into BPrivate::Network namespace

In order to prevent classes between libnetapi.so with the legacy API and
applications using the libnetservices.a library, the latter will have the
classes in a distinct namespace.

In the implementation, both libbnetapi.so and libnetservices.a will use the
same header and source files. If LIBNETAPI_DEPRECATED is defined during build,
the headers and source will have binary compatible behavior. Otherwise, the
classes and other objects will be put in the HaikuExt namespace.

In order to build the libbnetapi.so and libnetservices.a with the proper
build configuration, there is a stub `src/kits/net/libnetapi_deprecated` folder
that applies the special configuration to the source files.

Currently HaikuDepot, Webpositive, libshared.a and the http_streamer add on
use the compatible API in libbnetapi.so.

Change-Id: Ic73e9f271ef75749adda46f6f72e9a0b2851b461
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3667
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

libnetapi.so: make headers of deprecated classes private

These classes have been moved to the public API too soon, and they need some
more time to mature before they can be declared stable.

Change-Id: I9c52a8e6cc103922abde7a6b911fe0c3e6bf5700
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3665
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>