clients: NMSecretAgentSimple API fixups

nm_secret_agent_simple_*() functions should take an
NMSecretAgentSimple, not an NMSecretAgent.

The type macros were incorrectly validating against
NM_TYPE_SECRET_AGENT rather than NM_TYPE_SECRET_AGENT_SIMPLE.
clients: merge nm_secret_agent_simple_set_connection_path() into nm_secret_agent_simple_enable()

set_connection_path() is almost always called right before enable(),
and it's unclear why it would be called anywhere else.  So just
merge the two methods.
libnm: rename NMSecretAgent to NMSecretAgentOld

As with NMVpnPluginOld, rename NMSecretAgent to NMSecretAgentOld, to
leave the name "NMSecretAgent" open for a new-and-improved version in
NM 1.2.
libnm: merge branch 'secret-agent-old-bgo740345'

Rename NMSecretAgent to NMSecretAgentOld, to leave the name
"NMSecretAgent" open for a new-and-improved version in NM 1.2.

Also, a handful of NMSecretAgentSimple fixups.

https://bugzilla.gnome.org/show_bug.cgi?id=740345
Merge remote-tracking branches 'fd/lr/ipv6-tokens', 'fd/lr/veth-managed' and 'fd/lr/no-udev' into integration