doscmd heavily depends on struct sigcontext which luckily is mostly passed
between functions as usion regcontext_t. By redefining union regcontext_t in
terms of mcontext_t almost all bases are covered.
It also seems to me that doscmd was in a transitional state. The redundant
definitions made it difficult to get a clear overview and could easily cause
oversight. To make sure my changes were ok, I went as far as to complete the
transition. It was not exactly necessary, but I expect to have to come back
here some more ("whistle" if I'm wrong :-).
