configure: add option to disable automatic OpenSSL config loading

Sometimes it may be considered a security risk to load an external
OpenSSL configuration automatically inside curl_global_init(). The
configuration option --disable-ssl-auto-load-config disables this
automatism. The Windows build scripts winbuild/Makefile.vs provide a
corresponding option ENABLE_SSL_AUTO_LOAD_CONFIG accepting a boolean
value.

Setting neither of these options corresponds to the previous behavior
loading the external OpenSSL configuration automatically.

Fixes #2724
Closes #2791