build: rename FORCEDEBUG_CPPFLAGS to LIBZPOOL_CPPFLAGS

This is just a very small attempt to make it more obvious that these
flags aren't optional for libzpool-using programs, by not making it seem
like there's an option to say "well, I don't _want_ to force debugging".

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Issue #16476
Closes #16477