search improvements

ft_snippet()
- make utf8 algorithm default
- add workaround for utf8_substr() limitations, bug #891
- fix some indexes which missed out on conversion to utf8
  character counts
- minor improvements

idx_lookup()
- minor changes to wildcard matching code to improve performance
  (changes based on profiling results)

utf8
- specifically set mb_internal_coding to utf-8 when mb_string
  functions will be used.

darcs-hash:20060831003413-9b6ab-712021eda3c959ffe79d8d3fe91d2c9a8acf2b58.gz