Support checkHost for stunnel 5.15

[tats]

This patch will check the peer certificate subject when
mew-ssl-verify-level is non-zero with stunnel >=5.15 and
OpenSSL >=1.0.2.

cf. https://www.stunnel.org/NEWS.html

Version 5.21, 2015.07.27, urgency: MEDIUM

• More elaborate descriptions were added to the warning about
  using "verify = 2" without "checkHost" or "checkIP".

Version 5.15, 2015.04.16, urgency: LOW

• Added new service-level options "checkHost", "checkEmail" and
  "checkIP" for additional checks of the peer certificate subject.
  These options require OpenSSL version 1.0.2 or higher.

[kazu-yamamoto]

LGTM.

[kazu-yamamoto]

Merged. Thanks!