(tls-certtool-program, tls-certtool-program)

(tls-certtool-program, tls-certtool-program)
(tls-hostmismatch): New variables.
(tls-checktrust): New variable.  Check if GNU TLS complained about a
mismatch between the hostname provided in the certificate and the name
of the host connnecting to.
(open-tls-stream): Use them.  Check certificates against trusted root
certificates.