Changed some non fatal myisamchk error messages to warnings


myisam/myisamchk.c:
  Change error -> warning
sql/handler.cc:
  Add mutex around keybuff_size usage
sql/mysql_priv.h:
  Indentation update
sql/set_var.cc:
  Add mutex around longlong variable usage
sql/sql_parse.cc:
  Fix mutex handling in SHOW_VARIABLES
sql/sql_show.cc:
  Fix mutex handling in SHOW_VARIABLES