* transcode: follow the same logic for ab than vb. (ie for a small value

it's used as kb/s for higher ones it's b/s).
Just a remember, transcode options are:
acodec= mpga,a52
    ab (kb/s)
vcodec= mpgv,mp4v,h263,DIV1,DIV2,DIV3,I420,I422,I444,RV24,YUY2
    width
    height
    vb (kb/s)
    vt (b/s)
    deinterlace
    croptop
    cropbottom
    cropleft
    cropright
    keyint
    hq
    qmin (between 1 and 32)
    qmax (between 1 and 32)
only acodec and/or vcodec are mandatories (and it's a good idea to always
define vb,vt, and/or ab)