This allows a node built with CONFIG_VLAN to choose at run-time
whether vlans are allowed or not.  We build both pfilter rule-sets,
and according to the active vlan number we write one or the other.

The case "CONFIG_VLAN is not set" is handled by providing default
NULL pointers for beginning and end of the vlan rule-set.

As a side effect, I renamed the files and the functions in
the pfilter-builder.

This is how it works for me, with CONFIG_PFILTER_VERBOSE set,
so you see different rule-set  are used (29 rules vs. 16 rules):

    wrc# vlan set 0
    0 ("0") out of range
    Command "vlan": error -22
    wrc# vlan off
    fixing MAC adress in rule: use 22:33:44:55:66:77
    pfilter rule 00: 4.00000000
    pfilter rule 01: 1.e4466013
    [...]
    pfilter rule 28: 8.00000000
    current vlan: 0 (0x0)
    wrc# ip set 10.0.0.2
    IP-address: 10.0.0.2 (static assignment)

  (and I am now reachable by untagged frames)

    wrc# vlan set 10
    fixing MAC adress in rule: use 22:33:44:55:66:77
    fixing VLAN number in rule: use 10
    pfilter rule 00: 4.00000000
    pfilter rule 01: 1.e4466013
    [...]
    pfilter rule 15: 8.00000000
    current vlan: 10 (0xa)

  (and I am now reachable on vlan 10)

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

Conflicts:
	Makefile