(ITS#8155) add support for cacertdir with gnutls

[ryan@openldap.org]

Full_Name: Ryan Tandy
Version: 
OS: 
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Submission from: (NULL) (142.31.146.2)
Submitted by: ryan


GnuTLS recently (3.3.6) finally added an API for adding a directory of CA
files:

http://gnutls.org/manual/html_node/Core-TLS-API.html#gnutls_005fcertificate_005fset_005fx509_005ftrust_005fdir-1

https://www.happyassassin.net/2015/01/12/a-note-about-ssltls-trusted-certificate-stores-and-platforms/

Haven't actually tried it yet, but I think it should allow us to implement
cacertdir.

Just filing this for tracking right now, patch to come later.