Ticket 3069: fix of case-sensitive search

...if file encoding and locale are different.

Example: locale is KOI8-R, file encoding is UTF-8. Note: those encodings
are not same.

File content is following (in Russian):

йцукен
Йцукен

The difference is in first line char only: Й (lowercase letter) and й
(uppercase letter).  The search of Ê gives the result Й independently of
case sensitivity. й isn't found.
If switch "All charsets" on, nothing is found.

The main idea of fix is modification of search API to allow set of search
pattern charset and use if within search engine.
Old API:
mc_search_new (pattern, pattern_len);
New API:
mc_search_new (pattern, pattern_len, pattern_charset);

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
search: better support of --disable-charset

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>