helpers: add close_fds to exo

helpers: add close_fds to exo

Add close_fds to exo() for import-duplicity, and while we're
there, define readpipe in terms of exo(), since it's just a
simplification.

Default close_fds to True, since it doesn't look like we need to
preserve the open fds in our other calls.

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