MDEV-27126 my_getopt compares option names case sensitively

my_getopt compares option names case-sensitively, causing
"Unknown option" errors when users type mixed-case options like
wsrep_slave_UK_checks in lowercase wsrep_slave_fk_checks.

Made the comparison in the getopt_compare_strings() case-insensitive.

mysql-test/main/my_getopt_case_insensitive.opt

@@ -0,0 +1 @@
+--slOw_QuEry_loG=OFF

mysql-test/main/my_getopt_case_insensitive.result

@@ -0,0 +1,8 @@
+#
+# MDEV-27126: my_getopt compares option names case sensitively
+#
+# Check if the variable is set correctly from options
+SELECT @@GLOBAL.slow_query_log;
+@@GLOBAL.slow_query_log
+0
+# End of test.

mysql-test/main/my_getopt_case_insensitive.test

@@ -0,0 +1,8 @@
+--echo #
+--echo # MDEV-27126: my_getopt compares option names case sensitively
+--echo #
+
+--echo # Check if the variable is set correctly from options
+SELECT @@GLOBAL.slow_query_log;
+
+--echo # End of test.

mysql-test/suite/wsrep/r/wsrep_mixed_case_cmd_arg.result

@@ -0,0 +1,8 @@
+#
+# MDEV-27126: my_getopt compares option names case sensitively
+#
+# Check if the variable is set correctly from options
+SELECT @@GLOBAL.wsrep_slave_uk_checks;
+@@GLOBAL.wsrep_slave_uk_checks
+1
+# End of test.

mysql-test/suite/wsrep/t/wsrep_mixed_case_cmd_arg.cnf

@@ -0,0 +1,7 @@
+!include ../my.cnf
+
+[mysqld.1]
+wsrep-on=ON
+wsrep-provider=@ENV.WSREP_PROVIDER
+wsrep-cluster-address=gcomm://
+

mysql-test/suite/wsrep/t/wsrep_mixed_case_cmd_arg.opt

@@ -0,0 +1 @@
+--wsrep-slave-uk-checks=1

mysql-test/suite/wsrep/t/wsrep_mixed_case_cmd_arg.test

@@ -0,0 +1,11 @@
+--source include/have_innodb.inc
+--source include/have_wsrep_provider.inc
+--source include/have_binlog_format_row.inc
+--echo #
+--echo # MDEV-27126: my_getopt compares option names case sensitively
+--echo #
+
+--echo # Check if the variable is set correctly from options
+SELECT @@GLOBAL.wsrep_slave_uk_checks;
+
+--echo # End of test.

mysys/my_getopt.c

@@ -18,6 +18,7 @@
 #include <mysys_priv.h>
 #include <my_default.h>
 #include <m_string.h>
+#include <ctype.h>
 #include <stdlib.h>
 #include <mysys_err.h>
 #include <my_getopt.h>
@@ -962,7 +963,7 @@ my_bool getopt_compare_strings(register const char *s, register const char *t,
 
   for (;s != end ; s++, t++)
   {
-    if ((*s != '-' ? *s : '_') != (*t != '-' ? *t : '_'))
+    if ((*s != '-' ? tolower(*s) : '_') != (*t != '-' ? tolower(*t) : '_'))
       DBUG_RETURN(1);
   }
   DBUG_RETURN(0);