ODBC wide characters support

I committed patches for ODBC wide support. By default this feature is
disabled and code produced by default is backward "compatible" (apart
a small issue with SQLTables). Wide support is still marked as
experimental. It use internally a fixed UTF-8 encoding to maintain
compatibility with existing libTDS. It works very fine with Microsoft
servers where setting UTF-8 as client does not change the way server
send strings but have some issues with Sybase where server have to
convert every string to UTF-8 on wire. The patch still require small
updates to reduce and improve some conversions.

I was hesitant committing part of the patch for statistics but i
realized that statistics functions have problems for NULL data and
patch fixed this issue so I committed. There is still a small issue
using Sybase and wildcards for catalog (a feature supported by odbc
3).

I rewrote libiconv replacement code to support UCS-4 and make code
more readable. I does not recommend the use of replacement as
characters set implemented are very limited.

Regards
  Frediano Ziglio