charset: add support for Unicode normalisation with libicu

charset: add support for Unicode normalisation with libicu

This adds a direct conversion hook using libicu to perform NFC <-> NFD
conversion on UTF8 strings. The defined charset strings are "UTF8-NFC" and
"UTF8-NFD", to convert from one to the other the caller calls smb_iconv_open()
with the desired source and target charsets, eg

  smb_iconv_open("UTF8-NFD", "UTF8-NFC");

for converting from NFC to NFD.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>