feature: SSL/TLS cosocket API

* added new method sslhandshake() to the stream-typed cosocket objects.
* added new configuration directives lua_ssl_trusted_certificate,
  lua_ssl_verify_depth, lua_ssl_crl, lua_ssl_protocols, and
  lua_ssl_ciphers.

Thanks aviramc for the original patch in #290.
tests: fixed the test case added in commit 6ef948150.
tests: one minor Perl code style fix.
bugfix: memory leak cound happen when both cosocket connection pools and "lua_code_cache off" were used. this regression had appeared in the ssl cosocket patch, i.e., commit 5aed1966.
tests: suppressed a valgrind false positive in the nginx core.
bugfix: we did not set the c->pool->log properly where c is an upstream stream-typed cosocket connection. this regression had appeared in the ssl cosocket patch, i.e., commit 5aed1966.
tests: a minor style fix in a case.
tests: made a test regarding ngx.re.sub less possible to fail in slow testing modes.
tests: fixed a case regarding ngx.on_abort().
tests: use iscribblet.org instead of agentzh.com in tests because the latter is currently served by Chinese name servers, which are not quite stable.
feature: tcpsock:sslhandshake(): when the "session" argument is a boolean "false", then no SSL session userdata will be returned and only a boolean "true" value is returned when successful.
bugfix: builds without --with-http_ssl_module were broken. this regression had appeared in commit a6a0ed5.