BUG/MINOR: httpclient: fixed memory allocation for the SSL ca_file

BUG/MINOR: httpclient: fixed memory allocation for the SSL ca_file

The memory for the SSL ca_file was allocated only once (in the function
httpclient_create_proxy()) and that pointer was assigned to each created
proxy that the HTTP client uses.  This would not be a problem if this
memory was not freed in each individual proxy when it was deinitialized
in the function ssl_sock_free_srv_ctx().

  Memory allocation:
    src/http_client.c, function httpclient_create_proxy():
      1277:	if (!httpclient_ssl_ca_file)
      1278:		httpclient_ssl_ca_file = strdup("@system-ca");
      1280:	srv_ssl->ssl_ctx.ca_file = httpclient_ssl_ca_file;

  Memory deallocation:
    src/ssl_sock.c, function ssl_sock_free_srv_ctx():
      5613:	ha_free(&srv->ssl_ctx.ca_file);

This should be backported to version 2.6.