rgw: setup locks for libopenssl

rgw: setup locks for libopenssl

openssl <= 1.02 requires explicit callbacks for locking which libcurl doesn't
set. This causes random segmentation faults when openssl uses its global
structures across multiple threads. Providing a simple mutex lock/unlock
functions as a callback. We determine whether openssl is used for libcurl via
curl-config utility which should be installed as a part of our curl development
headers package. We also additionally check that the openssl version is < 1.1.0
which alleviates the need for these callbacks. In this patchset we have done the
following:

- move all curl related global init functionality under rgw::curl namespace
  since libcurl may need to set up various ssl libraries etc during its init
- introduce WITH_CURL_OPENSSL in cmake
  this checks the backend curl is deployed with using curl-config. Since curl
  devel is expected to be installed anyway, this binary should be available and
  can help identify the ssl backend curl was compiled with.
- we only setup the locks if beast/civetweb aren't terminated with ssl, since
  these libraries setup the locks anyway and we want to prevent double
  initialization of openssl. Also we pass in ~CURL_GLOBAL_SSL making curl not
  initialize openssl if civetwb/beast is initializing them. Unfortunately this
  flag is a noop from curl >= 7.57 wherein both the libraries will end up
  initializing openssl anyway, which might override certain settings like error
  strings if using openssl < 1.1

https://curl.haxx.se/libcurl/c/threadsafe.html
https://www.openssl.org/docs/man1.0.2/crypto/threads.html#DESCRIPTION

Fixes: http://tracker.ceph.com/issues/22951
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
Signed-off-by: Jesse Williamson <jwilliamson@suse.com>