MDEV-11356 Option skip-core-file does not work

remove ancient hard-coded treatment of --core-file. This enables
normal my_getopt behavior for the already existing sysvar

mysql-test/main/mysqld--help.result

@@ -173,7 +173,7 @@ The following specify which files/extra groups are read (specified before remain
  ALWAYS
  --console           Write error output on screen; don't remove the console
  window on windows.
- --core-file         Write core on errors.
+ --core-file         Write core on crashes
  -h, --datadir=name  Path to the database root directory
  --date-format=name  The DATE format (ignored)
  --datetime-format=name 
@@ -1441,6 +1441,7 @@ column-compression-zlib-wrap FALSE
 completion-type NO_CHAIN
 concurrent-insert AUTO
 console TRUE
+core-file TRUE
 date-format %Y-%m-%d
 datetime-format %Y-%m-%d %H:%i:%s
 deadlock-search-depth-long 15

mysql-test/suite/sys_vars/r/sysvars_server_embedded.result

@@ -645,13 +645,13 @@ COMMAND_LINE_ARGUMENT	REQUIRED
 VARIABLE_NAME	CORE_FILE
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	BOOLEAN
-VARIABLE_COMMENT	write a core-file on crashes
+VARIABLE_COMMENT	Write core on crashes
 NUMERIC_MIN_VALUE	NULL
 NUMERIC_MAX_VALUE	NULL
 NUMERIC_BLOCK_SIZE	NULL
 ENUM_VALUE_LIST	OFF,ON
 READ_ONLY	YES
-COMMAND_LINE_ARGUMENT	NULL
+COMMAND_LINE_ARGUMENT	OPTIONAL
 VARIABLE_NAME	DATADIR
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	VARCHAR

mysql-test/suite/sys_vars/r/sysvars_server_notembedded.result

@@ -645,13 +645,13 @@ COMMAND_LINE_ARGUMENT	REQUIRED
 VARIABLE_NAME	CORE_FILE
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	BOOLEAN
-VARIABLE_COMMENT	write a core-file on crashes
+VARIABLE_COMMENT	Write core on crashes
 NUMERIC_MIN_VALUE	NULL
 NUMERIC_MAX_VALUE	NULL
 NUMERIC_BLOCK_SIZE	NULL
 ENUM_VALUE_LIST	OFF,ON
 READ_ONLY	YES
-COMMAND_LINE_ARGUMENT	NULL
+COMMAND_LINE_ARGUMENT	OPTIONAL
 VARIABLE_NAME	DATADIR
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	VARCHAR

sql/mysqld.cc

@@ -6754,8 +6754,6 @@ struct my_option my_long_options[]=
   {"console", OPT_CONSOLE, "Write error output on screen; don't remove the console window on windows.",
    &opt_console, &opt_console, 0, GET_BOOL, NO_ARG, 0, 0, 0,
    0, 0, 0},
-  {"core-file", OPT_WANT_CORE, "Write core on errors.", 0, 0, 0, GET_NO_ARG,
-   NO_ARG, 0, 0, 0, 0, 0, 0},
 #ifdef DBUG_OFF
   {"debug", '#', "Built in DBUG debugger. Disabled in this build.",
    &current_dbug_option, &current_dbug_option, 0, GET_STR, OPT_ARG,
@@ -8542,9 +8540,6 @@ mysqld_get_one_option(int optid, const struct my_option *opt, char *argument)
   case (int) OPT_SKIP_HOST_CACHE:
     opt_specialflag|= SPECIAL_NO_HOST_CACHE;
     break;
-  case (int) OPT_WANT_CORE:
-    test_flags |= TEST_CORE_ON_SIGNAL;
-    break;
   case OPT_CONSOLE:
     if (opt_console)
       opt_error_log= 0;			// Force logs to stdout

sql/sys_vars.cc

@@ -531,10 +531,9 @@ bool check_has_super(sys_var *self, THD *thd, set_var *var)
   return false;
 }
 
-static Sys_var_bit Sys_core_file("core_file", "write a core-file on crashes",
-          READ_ONLY GLOBAL_VAR(test_flags), NO_CMD_LINE,
-          TEST_CORE_ON_SIGNAL, DEFAULT(IF_WIN(TRUE,FALSE)), NO_MUTEX_GUARD, NOT_IN_BINLOG,
-          0,0,0);
+static Sys_var_bit Sys_core_file("core_file", "Write core on crashes",
+          READ_ONLY GLOBAL_VAR(test_flags), CMD_LINE(OPT_ARG),
+          TEST_CORE_ON_SIGNAL, DEFAULT(IF_WIN(TRUE,FALSE)));
 
 static bool binlog_format_check(sys_var *self, THD *thd, set_var *var)
 {