A SET_IF_NOT_NULL() macro for optional return values

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.