Cleanup for some problems in tsearch patch:

Cleanup for some problems in tsearch patch:
- ispell initialization crashed on empty dictionary file
- ispell initialization crashed on affix file with prefixes but no suffixes
- stop words file was run through pg_verify_mbstr, with database
  encoding, but it's supposed to be UTF-8; similar bug for synonym files
- bunch of comments added, typos fixed, and other cleanup

Introduced consistent encoding checking/conversion of data read from tsearch
configuration files, by doing this in a single t_readline() subroutine
(replacing direct usages of fgets).  Cleaned up API for readstopwords too.

Heikki Linnakangas