allow RUN to send the environment to a local socket

RUN="socket:<name>" will send the environment in the kernel uevent
format to the named destination. Using the socket instead of the program
to pass the hotplug events to the HAL daemon, cuts down the running
time of udevstart from 0.8 to 0.4 seconds on my box.

  env -i ACTION=add DEVPATH=/block/hda/hda1 strace -s10000 ./udev block
  sendto(3, "add@/block/hda/hda1\0
    ACTION=add\0DEVPATH=/block/hda/hda1\0UDEV_LOG=3\0
    ID_TYPE=disk\0ID_MODEL=HTS726060M9AT00\0ID_SERIAL=MRH401M4G6UM9B\0
    ID_REVISION=MH4OA6BA\0ID_BUS=ata\0ID_PATH=pci-0000:00:1f.1-ide-0:0\0
    ID_FS_USAGE=other\0ID_FS_TYPE=swap\0ID_FS_VERSION=2\0
    ID_FS_UUID=9352cfef-7687-47bc-a2a3-34cf136f72e1\0
    ID_FS_LABEL=ThisIsSwap\0ID_FS_LABEL_SAFE=ThisIsSwap\0
    DEVNAME=/dev/hda1\0"

Signed-off-by: Kay Sievers <kay.sievers@suse.de>