exc: support input arg; return SubprocInfo from exc and exo

exc: support input arg; return SubprocInfo from exc and exo

Change exo to return a namedtuple so that you can refer to bits of the
result by name.

Base exc on exo so that it can support an input arg (even in python2),
and change both exc and exo to be a thinner wrapper around Popen so
that the arguments will behave roughly the same.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>