Slight change to the way the video device use is determined

The original CAM=`lsof $CAMERA` worked fine until I found out
that Bodhi Linux (and therefore perhaps others) returned quite
a few processes. So we are more acurate with this test and are
looking specifically for the 'mem' File Descriptor (FD) being
listed in the return of lsof

Tested on: Ubuntu 12.04, 12.10 and Bodhi 2.1.0

For reference. here's the output of lsof on Bodhi Linux with the
webcam in use as indicated by the 4th entry where the FD column
shows 'mem':

mjbrooks@dharma: ~ $ lsof /dev/video0
COMMAND    PID     USER   FD   TYPE DEVICE SIZE/OFF  NODE NAME
enlighten 2642 mjbrooks   13r   CHR   81,0      0t0 10634 /dev/video0
nm-applet 2710 mjbrooks   13r   CHR   81,0      0t0 10634 /dev/video0
pithos    2711 mjbrooks   13r   CHR   81,0      0t0 10634 /dev/video0
skype     2712 mjbrooks  mem    CHR   81,0          10634 /dev/video0
skype     2712 mjbrooks   13r   CHR   81,0      0t0 10634 /dev/video0
skype     2712 mjbrooks   66u   CHR   81,0      0t0 10634 /dev/video0
remmina   2713 mjbrooks   13r   CHR   81,0      0t0 10634 /dev/video0
enlighten 2722 mjbrooks   13r   CHR   81,0      0t0 10634 /dev/video0
terminolo 5005 mjbrooks   13r   CHR   81,0      0t0 10634 /dev/video0
chrome    6618 mjbrooks   13r   CHR   81,0      0t0 10634 /dev/video0
chrome    6623 mjbrooks   13r   CHR   81,0      0t0 10634 /dev/video0
terminolo 8241 mjbrooks   13r   CHR   81,0      0t0 10634 /dev/video0

It's not entirely clear why it's retuning these on Bodhi when Ubuntu
does not since Bodhi Linux is based on Ubuntu. Perhaps it's just
how e17 does things.
Merge pull request #35 from mjbrooks/busylight-lsof-fd

Slight change to the way the video device use is determined