Support CA verification in chroot'ed processes without direct file

Support CA verification in chroot'ed processes without direct file
access to the certificates.  SSL_CTX_load_verify_mem() is a frontend
to the new X509_STORE_load_mem() function that allows to load the CA
chain from a memory buffer that is holding the PEM-encoded files.
This function allows to handle the verification in privsep'ed code.

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