lib/strutil/strutil.c: cosmetics and minor refactoring.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
lib/strutil/strutilascii.c: cosmetics and minor refactoring.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
lib/strutil/strutil8bit.c: cosmetics and minor refactoring.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
lib/strutil/strutilutf8.c: cosmetics and minor refactoring.

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