Fixed compiler warnings (Mostly VC++):

- Removed not used variables
- Changed some ulong parameters/variables to ulonglong (possible serious bug)
- Added casts to get rid of safe assignment from longlong to long (and similar)
- Added casts to function parameters
- Fixed signed/unsigned compares
- Added some constructores to structures
- Removed some not portable constructs

Better fix for bug Bug #21428 "skipped 9 bytes from file: socket (3)" on "mysqladmin shutdown"
(Added new parameter to net_clear() to define when we want the communication buffer to be emptied)

sql/sql_db.cc

@@ -1302,7 +1302,6 @@ err:
 
 bool mysql_change_db(THD *thd, const char *name, bool no_access_check)
 {
-  int path_length;
   LEX_STRING db_name;
   bool system_db= 0;
 #ifndef NO_EMBEDDED_ACCESS_CHECKS