Make WiredTiger extension functions available to linked-in modules via
a new function wiredtiger_extension_api(WT_EXTENSION_API **pp).

Write some WT_EXTENSION_API documentation.

Remove WT_SESSION::msg_printf, add a new msg_printf extension function
instead, convert test/format to use it.

Quit using __UNUSED in the extension code, use explicit (void) casting,
it's cleaner.