xmlBufAvail() should return length without including a byte for NUL terminator

* buf.c:
- Return the number of bytes available in the buffer, but do not
  include a byte for the NUL terminator so that it is reserved.

* encoding.c:
* xmlIO.c:
- Remove code that subtracts 1 from the return value of
  xmlBufAvail().  It was implemented inconsistently anyway.
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