Added netctl-auto detection
More consistent approach to output
Make a directory for a unit file link before creating the link itself.

The '/etc/systemd/system/multi-user.target.wants' directory does not
necessarily exist before netctl is invoked.
Typo in a sample profile
Flush the interface configuration in ip_unset instead of bring_interface_down.

When switching networks in auto.action, the addresses are not flushed. This
is especially problematic with stateless ipv6 autoconfigutation, as invalid
IPs may stay around until their (potentially very long) lifetime has expired.

bring_interface_down is always called after ip_unset everywhere else, so
this change does not affect anything else.

V2: Make sure not to flush the link local address.
Restrict dhcpcd to IPv4

By default, dhcpcd requests leases for both versions of IPv6.
In the past, dhcpcd did not support IPv6, so this commits restores old
behavior.
Properly check existence of a connection type.

Substituting "/dev/null/nonexistent" for an empty $Connection,
of course, does generally work, but is somewhat ugly (esp. in that
the string is appended to $CONN_DIR, so the resulting path may
theoretically exist).

Instead, substitute an empty string for the complete path if $Connection
is empty: an empty string can never be readable.

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>