Problem: Too confusing error message when cannot convert
between string and column character sets on INSERT and UPDATE.
Fix: producing a better error message, instead of "Data too long"
in such cases
Additional changes: Adding "DROP TABLE IF EXISTS" into several
tests to be safe against failures in previous tests. 


mysql-test/include/strict_autoinc.inc:
  Adding DROP TABLE to be safe against previous tests failure.
mysql-test/r/ctype_recoding.result:
  Fixing test results
mysql-test/r/ctype_utf8.result:
  Fixing test results
mysql-test/r/fulltext.result:
  Fixing test results
mysql-test/r/strict_autoinc_1myisam.result:
  Adding DROP TABLE to be safe against previous tests failure.
mysql-test/r/strict_autoinc_2innodb.result:
  Adding DROP TABLE to be safe against previous tests failure.
mysql-test/r/strict_autoinc_3heap.result:
  Adding DROP TABLE to be safe against previous tests failure.
mysql-test/r/strict_autoinc_4bdb.result:
  Adding DROP TABLE to be safe against previous tests failure.
mysql-test/r/strict_autoinc_5ndb.result:
  Adding DROP TABLE to be safe against previous tests failure.
sql/field.cc:
  - producing better error messages than
    "DATA TRUNCATED" or "DATA TOO LONG" (in strict mode)
    in case of "not well formed source" and
    "cannot convert to field character set"
  - Performance improvements: copying directly to
    the target, instead of using an intermediate
    String.
  - Moving duplicate code into report_data_too_long() function.
sql/sql_string.cc:
  Adding a new function to convert strings between character sets,
  but not more than "nchar" characters - a helper function for
  Field_string::store(), Field_varstring::store() and Field_blob::store().
sql/sql_string.h:
  Adding new function prototype.