mg: handle C-u in M-! and M-|

mg: handle C-u in M-! and M-|

With the C-u modifier, these commands (respectively shell-command
and shell-command-on-region) will operate in-place instead of opening
a special buffer with the result.

ok and lots of feedback from florian@

(signature for iomux and preadin changed after the ok -- the buffer
pointer was no longer needed)