vlc_stream: clean data skipping calls with vlc_stream_Read(NULL)

• handle vlc_killed() check (on EINTR or any error)
• handle continue on EAGAIN and EINTR (no pacing on the caller side)
• handle reading in parts (it won't read more than 256 bytes at a time)

All of these are missing from the replaced vlc_stream_Read(NULL) calls.

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v4:

• pass a uint64_t to vlc_stream_Skip() as suggested
• clarify what kind of errors can occur, copied from vlc_stream_Read()
• add some type cleaning commits before adding vlc_stream_Skip()

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v5:

• actually removed vlc_stream_Skip() and use vlc_stream_Read() instead.
• made all the calls consistent so we can easily find them and/or replace them.