MDL-43679 core_useragent: add basic MS Word detection

Basic detection for MS Word's internal link checker.  This is not perfect,
as older versions use IE6/7 user agent string with no identifying marks,
and newer versions don't necessarily identify themselves as Word (just as
Office).  Version information is not reliably available, even when there
is sufficient other information to identify it as Word's link checker.
This is also based on a small sample of Word / OS version pairings, so
there may be some which are identifiable but not currently covered.
MDL-43679 core_useragent: add test cases for MS Word

The new is_msword() check should always return false for regular browsers.
It should return true for known MS Word, but false for known MS Outlook.
MDL-43679 core: Never use HTTP redirect when the user agent is MS Word