Backport my_strntod() from 5.0

Change string->float conversion to delay division as long as possible.
This gives us more exact integer->float conversion for numbers of type '123.45E+02' (Bug #7740)




client/mysql.cc:
  Fix wront usage of charset (found during review of pushed code)
include/m_string.h:
  Backported my_strtod() from 5.0
mysql-test/mysql-test-run.sh:
  Run also mysql_client_test with --debug
mysql-test/r/ps_1general.result:
  Safety fix (if mysql_client_test.test fails)
mysql-test/r/type_float.result:
  More test
mysql-test/t/mysql_client_test.test:
  Comments for what to do if this test fails
mysql-test/t/ps_1general.test:
  Safety fix (if mysql_client_test.test fails)
mysql-test/t/type_float.test:
  More test to better test new strtod() function
  Test also bug #7740 (wrong comparsion between integer and float-in-integer-range)
sql/field.cc:
  Backport my_strntod() from 5.0
sql/item.cc:
  Backport my_strntod() from 5.0
sql/item.h:
  Backport my_strntod() from 5.0
sql/item_func.h:
  Backport my_strntod() from 5.0
sql/item_strfunc.cc:
  Backport my_strntod() from 5.0
sql/item_sum.cc:
  Backport my_strntod() from 5.0
sql/item_sum.h:
  Backport my_strntod() from 5.0
sql/procedure.h:
  Backport my_strntod() from 5.0
strings/ctype-simple.c:
  Backport my_strntod() from 5.0
strings/ctype-ucs2.c:
  Backport my_strntod() from 5.0
strings/strtod.c:
  Backport my_strntod() from 5.0
  Change conversion to delay division as long as possible.
  This gives us more exact integer-> float conversion for numbers of type '123.45E+02'

mysql-test/r/ps_1general.result

@@ -1,5 +1,6 @@
 drop table if exists t5, t6, t7, t8;
 drop database if exists mysqltest ;
+drop database if exists client_test_db;
 test_sequence
 ------ basic tests ------
 drop table if exists t1, t9 ;