move fix_bootif to net-lib

This might be useful to other parts of dracut; move it to net-lib.sh.
net-genrules: accept MAC address for $iface, cleanups

A MAC address is a unique identifier for a particular network interface.

We can use the MAC to generate udev rules to bring up that interface,
like we currently do with BOOTIF.

This patch allows interfaces to be specified as a MAC address, either
in the usual colon-separated form or the PXE-style dash-separated form.

(The latter is more useful on the commandline, since it allows for
arguments like: "ip=77-77-6f-6f-64-73:dhcp")

This is useful since it's common for a user who is booting a new OS for
the first time to know the MAC of the device, but not know what the
kernel name will be.
Don't explode when mixing BOOTIF and ip=

Now that we can use a MAC as a device identifier, we can just bring up
the device specified by BOOTIF as a normal interface.

So instead of ignoring everything but BOOTIF, we'll put BOOTIF in the
IFACES list and bring it up as normal, defaulting to DHCP if nothing
else is specified.

We can also handle anaconda-style 'ksdevice=bootif' this way.
net-lib: add find_iface_with_link()

Add a function to look at the system network interfaces and return the
first one it finds with an active link (if any.)
net-lib: add configured_ifaces()

configured_ifaces is a function that returns the names of each interface
that the user wanted configured.

Currently, this is accomplished by reading the list from
/tmp/net.ifaces. But if we want to allow the user to specify an
interface by its MAC address or IP or something, we need a function that
will read the cache and convert the MACs etc. to names.

(Obviously this conversion only works once udev starts, so it will warn
you if you try it too early.)
net-lib: fix set_ifname()

/sys/class/$name$num isn't ever going to work; /sys/class/net/$name$num
(e.g. /sys/class/net/ibft0) should work right.
systemd/module-setup.sh: mkdir /etc/systemd
test/TEST-50-MULTINIC: fixed debug output
network: refix BOOTIF logic
test/TEST-50-MULTINIC: fix interface test
90lvm/module-setup.sh: redirect error message of lvs to /dev/null

Signed-off-by: Baoquan He <bhe@redhat.com>