Bunch of tsearch fixes and cleanup

Fixes the following bugs:
- ispell initialization crashed on empty dictionary file
- ispell initialization crashed on affix file with prefixes but no suffixes
- stop words file was ran through pg_verify_mbstr, with database
encoding, but it's later interpreted as being UTF-8. Now verifies that
it's UTF-8, regardless of database encoding.

Other changes:

- readstopwords now sorts the stop words after loading them. Removed the
separate sortstopwords function.

- readstopwords calls recode_and_lowerstr directly, instead of using the
"wordop" function pointer in StopList struct. All callers used
recode_and_lowerstr anyway, so this simplifies the code a little bit. Is
there any external dictionary implementations that would require
different behavior?

- bunch of comments added, typos fixed, and other cleanup

The code still needs lots of love, but it's a start...

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com