Handle ISC_MEM_DEFAULTFILL consistently

Handle ISC_MEM_DEFAULTFILL consistently

Contrary to what the documentation states, memory filling is only
enabled by --enable-developer (or by setting -DISC_MEM_DEFAULTFILL=1) if
the internal memory allocator is used.  However, the internal memory
allocator is disabled by default, so just using the --enable-developer
build-time option does not enable memory filling (passing "-M fill" on
the named command line is necessary to actually enable it).  As memory
filling is a useful tool for troubleshooting certain types of bugs, it
should also be enabled by --enable-developer when the system allocator
is used.

Furthermore, memory-related preprocessor macros are handled in two
distinct locations: lib/isc/include/isc/mem.h and bin/named/main.c.
This makes the logic hard to follow.

Move all code handling the ISC_MEM_DEFAULTFILL preprocessor macro to
lib/isc/include/isc/mem.h, ensuring memory filling is enabled by the
--enable-developer build-time switch, no matter which memory allocator
is used.