Mark stable/9 as 9.3-PRERELEASE to indicate the code slush
is now in effect for the 9.3-RELEASE cycle.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation
Now that stable/9 is in 9.3 mode, switch default manual page
version to 9.3.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation