BUG/MINOR: ssl: SSL_CERT_DIR environment variable doesn't affect haproxy

BUG/MINOR: ssl: SSL_CERT_DIR environment variable doesn't affect haproxy

The documentation of @system-ca specifies that one can overwrite the
value provided by the SSL Library using SSL_CERT_DIR.

However it seems like X509_get_default_cert_dir() is not affected by
this environment variable, and X509_get_default_cert_dir_env() need to
be used in order to get the variable name, and get the value manually.

This could be backported in every stable branches. Note that older
branches don't have the memprintf in ssl_sock.c.