Fix for BUG#15921: DROP TRIGGER - can't be drop trigger created

in older version.

The problem is that TRN-files created in "old" versions contain
junk in trigger_table field, which is not acceptable in "new" versions.


sql/parse_file.cc:
  1. Made parse_escaped_string() available globally;
  2. Polishing: fixing the comments.
sql/parse_file.h:
  Made parse_escaped_string() available globally.
sql/sql_trigger.cc:
  1. Polishing
  2. Removed declaration of global handle_old_incorrect_sql_modes()
     since it is not used anymore.
  3. Added a hook to support invalid "trigger_table" field in TRN-file
     from old versions.

sql/parse_file.cc

@@ -533,7 +533,7 @@ parse_string(char *ptr, char *end, MEM_ROOT *mem_root, LEX_STRING *str)
   read escaped string from ptr to eol in already allocated str
 
   SYNOPSIS
-    parse_escaped_string()
+    read_escaped_string()
     ptr		- pointer on string beginning
     eol		- pointer on character after end of string
     str		- target string
@@ -604,7 +604,7 @@ read_escaped_string(char *ptr, char *eol, LEX_STRING *str)
     #	- pointer on symbol after string
 */
 
-static char *
+char *
 parse_escaped_string(char *ptr, char *end, MEM_ROOT *mem_root, LEX_STRING *str)
 {
   char *eol= strchr(ptr, '\n');
@@ -622,7 +622,7 @@ parse_escaped_string(char *ptr, char *end, MEM_ROOT *mem_root, LEX_STRING *str)
   parse '' delimited escaped string
 
   SYNOPSIS
-    parse_escaped_string()
+    parse_quoted_escaped_string()
     ptr		- pointer on string beginning
     end		- pointer on symbol after parsed string end (still owned
 		  by buffer and can be accessed