* encoding.c, include/libxml/encoding.h: Enhanced the handling of UTF-16,
  UTF-16LE and UTF-16BE encodings.  Now UTF-16 output is handled internally
  by default, with proper BOM and UTF-16LE encoding.  Native UTF-16LE and
  UTF-16BE encoding will not generate a BOM on output, and will be
  automatically recognized on input.
* test/utf16lebom.xml, test/utf16bebom.xml, result/utf16?ebom*: added
  regression tests for above.