Added --tls-keydirectory option.
Refactored directory access mechanism into own functions.
configure_driver: Added DC_TLS_CERTIFICATE to select a certificate.
GnuTLS: Implemented --tls-keydirectory and DC_TLS_CERTIFICATE.
GnuTLS: Reimplemented support for certificate chains.
GnuTLS: Cleanup all keys on shutdown.
GnuTLS: Follow struct naming convention.
TLS: Moved tls_opendir and tls_readdir to pkg-tls.c
OpenSSL: Implemented --tls-keydirectory and DC_TLS_CERTIFICATE.
GnuTLS: Support for versions of GnuTLS < 2.11.
Added garbage collection support for TLS structures.
Fixed a memory leak in configure_driver(DC_TLS_CERTIFICATE, ...).