2. added automatic quotation of keywords in SHOW CREATE TABLE


mysql-test/r/show_check.result:
  added tests for 
  bug #2593 "SHOW CREATE TABLE doesn't properly double quotas"
  and for automatic quotation of keywords
mysql-test/t/show_check.test:
  added tests for 
  bug #2593 "SHOW CREATE TABLE doesn't properly double quotas"
  and for automatic quotation of keywords
sql/mysql_priv.h:
  added declaration of function is_keyword
sql/sql_lex.cc:
  added implementation of function is_keyword
sql/sql_show.cc:
  changed function append_identifier and it's subfunctions
  1. fixed bug @2593 "SHOW CREATE TABLE doesn't properly double quotes"
  2. added automatic quotation of keywords