apps/openssl.c: Adapt to enable tracing output

Use the environment variables OPENSSL_TRACE to determine what's going
to be enabled.  The value of this variables is a comma separated list
of trace and debugging names, which correspond to the trace category
macros defined in include/openssl/trace.h.

For example, setting OPENSSL_DEBUG=TRACE,SSL will enable debugging output
for the types OSSL_TRACE_CATEGORY_TRACE and OSSL_TRACE_CATEGORY_SSL.

This also slightly changes the handling of the prefix method in
apps/apps.c.  This is for the better, as the prefix method pointer was
unneccessarily stored in two places.

Co-authored-by: Dr. Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8198)