xlog filename formatting functions in recovery

Hello,

I've noticed recently that I can't seem to use the convenient xlog
filename formatting functions while I'm in a standby.  I don't see an
incredibly obvious reason why that is the case, so here's a patch that
simply removes the ban on being able to call these formatting
functions.

Perhaps I'm missing something, but it feels like this is an oversight
from the days when there was no useful function to call that formatted
a X/X-type record on a standby.

-- 
fdr