Removed duplicated code (in pp.c and mg.c) by introducing

Removed duplicated code (in pp.c and mg.c) by introducing
do_vecget().  NOTE: the calling convention of do_vecset()
changes, too: the `offset' that is assigned to LvTARGOFF(TARG)
in pp_vec() is no more multiplied by `size' in pp_vec(),
the multiplication is now done in do_vecset().

Also fix a cpp thinko in change #4002.

p4raw-link: @4002 on //depot/cfgperl: 24db6c0d56fddf85ee587fc1cb1dbce678fa6a8c

p4raw-id: //depot/cfgperl@4004