Panic strings

[Constantine Sapuntzakis <csapuntz_(_at_)_tma-1_(_dot_)_lcs_(_dot_)_mit_(_dot_)_edu>]

The panic string (panicstr) is currently being set to the format string
passed to panic. This is done (I assume) so that we don't have to call
malloc in panic to allocate a buffer for the final panic string. 

I believe a reasonable alternative would be to have fixed sized
panic buffer and to snprintf into it. 256 characters seems like an
exceedingly reasonable size.

Any opinions?

-Costa