apps/speed.c: Wait for generated children

apps/speed.c: Wait for generated children

In multi-mode, speed fork()s off several children but does not wait for them.
On Linux, this leads to wrong accounting information of getrusage used by
tools to extract running time and page faults.

Wait for every children and check the return code and termination signal.

Signed-off-by: Juergen Christ <jchrist@linux.ibm.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19093)