Oracle: fix ORA-01442 error during installation

Oracle does not allow to add a NOT NULL constraint if it's already
present; it will just keep the existing constraint if a column is
altered without mentioning NOT NULL. MySQL on the other hand will
actually remove the constraint if NOT NULL is not specified again.
Fix PHP warning in db_table_exists()

If db_get_table_list() does not return an array, PHP will issue a
'Invalid argument supplied for foreach' warning.
HTML API: revise order of elements in HEAD section

The HTML specification [1] does not specify a strict order for the
elements within the HEAD section, other than Content-Type appearing
within the first 1024 bytes of the document.

Microsoft / Internet Explorer has stricter guidelines when it comes to
defining compatibility mode [2]: "The X-UA-Compatible header [...] must
appear in the header of the webpage (the HEAD section) before all other
elements except for the title element and other meta elements."

This commit reorganizes html_page_top1() function to:
- define Content-Type as the HEAD's first element
- allow users to set X-UA-Compatible in $g_meta_include_file
- include CSS after defining the TITLE, together with other LINK tags

Fixes #17176

[1] http://www.w3.org/TR/html5/document-metadata.html
[2] http://msdn.microsoft.com/en-us/library/jj676915%28v=vs.85%29.aspx