avoid ulong sysvars, prefer uint or ulonglong

ulong is not portable - different width on different architectures

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

@@ -1793,7 +1793,7 @@ GLOBAL_VALUE	1024
 GLOBAL_VALUE_ORIGIN	COMPILE-TIME
 DEFAULT_VALUE	1024
 VARIABLE_SCOPE	GLOBAL
-VARIABLE_TYPE	BIGINT
+VARIABLE_TYPE	INT UNSIGNED
 VARIABLE_COMMENT	Maximum length considered for digest text.
 NUMERIC_MIN_VALUE	0
 NUMERIC_MAX_VALUE	1048576

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

@@ -1947,7 +1947,7 @@ GLOBAL_VALUE	1024
 GLOBAL_VALUE_ORIGIN	COMPILE-TIME
 DEFAULT_VALUE	1024
 VARIABLE_SCOPE	GLOBAL
-VARIABLE_TYPE	BIGINT
+VARIABLE_TYPE	INT UNSIGNED
 VARIABLE_COMMENT	Maximum length considered for digest text.
 NUMERIC_MIN_VALUE	0
 NUMERIC_MAX_VALUE	1048576

sql/mysqld.cc

@@ -520,7 +520,7 @@ ulong binlog_cache_use= 0, binlog_cache_disk_use= 0;
 ulong binlog_stmt_cache_use= 0, binlog_stmt_cache_disk_use= 0;
 ulong max_connections, max_connect_errors;
 ulong extra_max_connections;
-ulong max_digest_length= 0;
+uint max_digest_length= 0;
 ulong slave_retried_transactions;
 ulonglong slave_skipped_errors;
 ulong feature_files_opened_with_delayed_keys;

sql/mysqld.h

@@ -187,7 +187,7 @@ extern ulong query_cache_limit;
 extern ulong query_cache_min_res_unit;
 extern ulong slow_launch_threads, slow_launch_time;
 extern MYSQL_PLUGIN_IMPORT ulong max_connections;
-extern ulong max_digest_length;
+extern uint max_digest_length;
 extern ulong max_connect_errors, connect_timeout;
 extern my_bool slave_allow_batching;
 extern my_bool allow_slave_start;

sql/sys_vars.cc

@@ -1346,7 +1346,7 @@ static Sys_var_ulong Sys_max_connect_errors(
        VALID_RANGE(1, UINT_MAX), DEFAULT(MAX_CONNECT_ERRORS),
        BLOCK_SIZE(1));
 
-static Sys_var_long Sys_max_digest_length(
+static Sys_var_uint Sys_max_digest_length(
        "max_digest_length", "Maximum length considered for digest text.",
        PARSED_EARLY READ_ONLY GLOBAL_VAR(max_digest_length),
        CMD_LINE(REQUIRED_ARG),