A SET_IF_NOT_NULL() macro for optional return values

The SET_IF_NOT_NULL() macro avoids a fair amount of tedious boilerplate, checking pointer parameters to see if they're non-NULL and updating them if they are. The macro was already in the dns_zone unit, and this commit moves it to the <isc/util.h> header.

I have included a Coccinelle semantic patch to use SET_IF_NOT_NULL() where appropriate. The patch needs an #include in openssl_shim.c in order to work.