change 10 to a #define'd constant


myisam/myisamdef.h:
  use a constant with a difficult-to-type name instead of two-digit number :)
sql/ha_myisam.cc:
  use a constant with a difficult-to-type name instead of two-digit number :)
sql/sql_insert.cc:
  always call start_bulk_insert (it performs all checks itself)
  removed spurious semicolon at the end of the "if" :)
  added a clarifying comment