Modernize UTF-8/UCS-2 conversion code

Modernize UTF-8/UCS-2 conversion code

Remove unused entry points as we only need to convert between
little-endian UCS-2 byte buffers and UTF-8.  Rename and simplify the
remaining two function contracts.  Avoid pointer alignment and
endianness issues by operating on byte buffers and using store_16_le()
and load_16_le().  Avoid two-pass operation using k5buf.

[ghudson@mit.edu: simplified code using k5buf; simplified function
names and contracts; rewrote commit message]