Syntax modified to allow statements:
               ALTER TABLE ADD/DROP COLUMN
               ALTER TABLE ADD/DROP INDEX
               ALTER TABLE ADD/DROP FOREIGN KEY
               ALTER TABLE ADD/DROP PARTITION
               ALTER TABLE CHANGE COLUMN
               ALTER TABLE MODIFY COLUMN
               DROP INDEX
         to have IF (NOT) EXISTS options.
         Appropriate implementations added to mysql_alter_table().
      
      per-file comments:
        mysql-test/r/alter_table.result
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              test result updated.
        mysql-test/r/fulltext.result
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
        mysql-test/r/partition.result
              test result updated.
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
        mysql-test/t/alter_table.test
              tests added.
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
        mysql-test/t/fulltext.test
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              tests added.
        mysql-test/t/partition.test
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              tests added.
        sql/field.cc
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              create_if_not_exists field added.
        sql/field.h
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              create_if_not_exists field added.
        sql/partition_info.h
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              has_unique_name made public.
        sql/sp_head.cc
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
        sql/sql_class.cc
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              create_if_not_exists inited.
        sql/sql_class.h
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              create_if_not_exists inited.
        sql/sql_lex.cc
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              check_exists inited.
        sql/sql_lex.h
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              check_exists inited.
        sql/sql_parse.cc
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
        check_exists inited.
        sql/sql_table.cc
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              handle_if_exists_options() added.
              it's called in mysql_alter_table().
        sql/sql_trigger.cc
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              check_exists instead of drop_if_exists.
        sql/sql_view.cc
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              check_exists instead of drop_if_exists.
        sql/sql_yacc.yy
      MDEV-318 IF (NOT) EXIST clauses for ALTER TABLE (MWL #252).
              sintax modified.