Add X509_STORE_load_mem() to load certificates from a memory buffer

Add X509_STORE_load_mem() to load certificates from a memory buffer
instead of disk.  OpenSSL didn't provide a built-in API from loading
certificates in a chroot'ed process that doesn't have direct access to
the files.  X509_STORE_load_mem() provides a new backend that will be
used by libssl and libtls to implement such privsep-friendly
functionality.

Adopted for LibreSSL based on older code from relayd (by pyr@ and myself)
With feedback and OK bluhm@