Minor review of commit c823d1520d037d0ef2cd2e6e4120cb13159d50ff.

Change ifxStartTransaction(), ifxCommitTransaction() and
ifxRollbackTransaction() to use ifxGetSqlStateMessage() instead
using their own handling of SQLSTATE via strncmp(), which looks
much cleaner. Additional code refactoring.

While at it, make ifxCloseConnection() to throw errors on
various conditions, e.g. uninitialized connection cache or
errors during closing an Informix connection. Formerly everything
was ignored, which looks dumb to me.

Also adjust some misleading or wrong comments.