Rework and make DEBUG macros consistent.

Rework and make DEBUG macros consistent.

Remove unused -DCONF_DEBUG and -DBN_CTX_DEBUG.

Rename REF_PRINT to REF_DEBUG for consistency, and add a new
tracing category and use it for printing reference counts.

Rename -DDEBUG_UNUSED to -DUNUSED_RESULT_DEBUG

Fix BN_DEBUG_RAND so it compiles and, when set, force DEBUG_RAND to
be set also.

Rename engine_debug_ref to be ENGINE_REF_PRINT also for consistency.

Fixes #15357

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15353)