std: more reliable HTTP and TLS networking

* std.Io.Reader: fix confused semantics of rebase. Before it was
  ambiguous whether it was supposed to be based on end or seek. Now it
  is clearly based on seek, with an added assertion for clarity.

* std.crypto.tls.Client: fix panic due to not enough buffer size
  available. Also, avoid unnecessary rebasing.

* std.http.Reader: introduce max_head_len to limit HTTP header length.
  This prevents crash in underlying reader which may require a minimum
  buffer length.

* std.http.Client: choose better buffer sizes for streams and TLS
  client. Crucially, the buffer shared by HTTP reader and TLS client
  needs to be big enough for all http headers *and* the max TLS record
  size. Bump HTTP header size default from 4K to 8K.

fixes #24872

I have noticed however that there are still fetch problems

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