Handle inability to create AFALG socket

Some Linux platforms have a suitably recent kernel to support AFALG, but
apparently you still can't actually create an afalg socket. This extends
the afalg_chk_platform() function to additionally check whether we can
create an AFALG socket. We also amend the afalgtest to not report a
failure to load the engine as a test failure. A failure to load is almost
certainly due to platform environmental issues, and not an OpenSSL problem.

RT 4434

Reviewed-by: Andy Polyakov <appro@openssl.org>