Better explanation message for dmalloc.

Also more insertion of parenthesis to handle struct members called
'free'.

You can now get useful dmalloc output, as long as it is compatible
with your C library.  On RH7.1 it looks like you have to rebuild
dmalloc to allow free(0) by default, because something in libcrypt
does that. (sigh)
(This used to be commit 391cbb690196537c8b6292b42c2e27408cc7e249)