Tidy up "diag_logs_settings.php" XHTML

Add "closehead" PHP variable and close the HEAD statement
Add CDATA to script statement
Add SUMMARY to table statement
Tidy up HTML Boolean operators
Close BR and INPUT statements
Remove duplicate closing table cell statement
Merge pull request #840 from ExolonDX/branch_15

Tidy up "diag_logs_settings.php" XHTML