Deprecate watchdog()

I recently was confused by:

• The UI page is called 'Recent log messages'
• The module is called 'Database Logging' (dblog)
• The API function for adding messages is called watchdog()

My research took me to this page which explains that the term 'watchdog' comes from the name of the predecessor to our dblog module from Drupal 4 & 5!

Since we more-or-less use the terms 'log/logging' now (in both the UI and the module name), I propose deprecating the watchdog*() functions and creating more generic and consistent log*() functions instead.