util/perl/OpenSSL/config.pm: refactor map_guess()

util/perl/OpenSSL/config.pm: refactor map_guess()

map_guess() is now table driven, just like get_system().
Additionally, it now takes a config hash table and returns one of its
own.  This way, 'Configure' can pass whatever it has already found to
OpenSSL::config::get_platform(), and easily merge the returned hash
table into its %config.

This also gets rid of variables that we no longer need.  That includes
$PERL and all the $__CNF_ environment variables.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11230)