ifup places illegally named variables into the environment

[LaMont Jones]

When dhclient brings up interface eth0.55, the environment passed to scripts includes:
    IFUPDOWN_eth0=parent-lock
    IFUPDOWN_eth0.55=post-up
    IFACE=eth0.55
    LOGICAL=eth0.55
    ADDRFAM=inet
    METHOD=dhcp
    MODE=start
    PHASE=post-up
    VERBOSITY=0
    PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

IFUPDOWN_eth0.55 is not a valid name for an environment variable.