NetServices: Rework parsing of content-length and move to HttpParser

This fixes a bug in the HttpAuthTest, and in general, moves responsibility of determining
the shape and size of the content to the right place.

Still to do is to fix the case where there really is a variable length content. This will
now probably error out as a connection closed error.

Change-Id: I13c20028e834cc9af8c7bc1d7d7613bf60838e64

headers/private/netservices2/HttpFields.h

@@ -115,6 +115,7 @@ public:
 	// Querying
 			ConstIterator		FindField(const std::string_view& name) const noexcept;
 			size_t				CountFields() const noexcept;
+			size_t				CountFields(const std::string_view& name) const noexcept;
 
 	// Range-based iteration
 			ConstIterator		begin() const noexcept;