plugin_dir option backported from 5.1

per-file messages:
  sql/mysql_priv.h
    Bug#37428 Potential security issue with UDFs - linux shellcode execution.
    
    opt_plugin_dir and opt_plugin_dir_ptr declared.
  sql/mysqld.cc
    Bug#37428 Potential security issue with UDFs - linux shellcode execution.
    
    'plugin_dir' option added
  sql/set_var.cc
    Bug#37428 Potential security issue with UDFs - linux shellcode execution.
    
    'plugin_dir' option added.
  sql/sql_udf.cc
    Bug#37428 Potential security issue with UDFs - linux shellcode execution.
    
    opt_plugin_dir added to the udf->dl path. Warn if it's not specified.
  sql/unireg.h
    Bug#37428 Potential security issue with UDFs - linux shellcode execution.
    
    PLUGINDIR defined.