Support : Fixes for Relative URL Handling

When URLs combine a base URL with a relative part, the relative part's
path component was being pre-processed. This removed any ".." from the
path and in some cases in the unit test cases, the ".." should have been
retained and then only later applied to the base URL. This changes
fixes this so that the relative part is not pre-processed and is applied
with it's path in a raw state.

Completes Fixes for #14377
Change-Id: I9cebb8599889494e11f40a3b54c87ebca3ed1a21
Reviewed-on: https://review.haiku-os.org/529
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

BUrl: Remove HAIKU_TARGET_PLATFORM_HAIKU from main header.

These are really only defined during the build of Haiku itself,
so we don't want them in a system header. Since none of these
functions are virtual, leaving them as declared but not defined
should be fine.

Support : Validation for Host Portion of URL

Partly Fixes #14377

Change-Id: Ia611d3653d2c16c6dcdc48ce57bd61bb6e6db366
Reviewed-on: https://review.haiku-os.org/476
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

move BUrl to the support kit

It is used by the media kit, which created a dependency from libmedia to
libbnetapi to openssl.
It is not entirely specific to the network kit, there are some use cases
that don't involve network at all.