g_tls_certificate_list_new_from_file() was leaking the file contents,
and GSource was leaking the GSourcePrivate structure that got
created when using child sources.
Add a method to verify a certificate against a CA; this can be used
for apps that need to test against non-default CAs.
Also make the GTlsCertificate::issuer property virtual
This adds an extension point for TLS connections to gio, with a
gnutls-based implementation in glib-networking.
Full TLS support is still a work in progress; the current API is
missing some features, and parts of it may still be changed before
2.28.
https://bugzilla.gnome.org/show_bug.cgi?id=588189
