Improve invisible errors in code

[fho4abcd]

Errors were found by checking the source of the html pages and checking for errors in the web toolkit for developers.

• change_module.php: was not used and is now deleted
• header.php: simplify the DOCTYPE statement. This removes a warning.
• homepage.php: Removed unused Modulo + html in correct order + clean code
• institutional_info.php:
  • Removed unused functions Modulo(gave errors) + ChangeLang + AbrirAyuda
  • rename Cambiarbase: avoid confusion with duplicates
  • display language menu value in "best" characterset
  • remove BOM (Byte Order Mark) if present on first line of language file. First language entry is shown if selected
  • cleanup code & html
• menu_mantimiento.php: removed unused iframe