Negotiation with no dgsh I/O should produce a sensible default

[dspinellis]

• [X] When no dgsh is present in input or output, dgsh_negotiate should succeed if possible (if number of inputs and outputs is less than two) , returning stdin and stdout file descriptors as required.

• [X] Although currently the code is supposed to display an error in this case, it doesn't. (This will not be needed after the fix, but it may indicate another bug.)

  [dds@stereo ~]$ head /usr/share/dict/words | ~/libexec/dgsh/sort -r
  [dds@stereo ~]$

• [X] When input or output fds is > 1 and there is no dgsh client on that side, the negotiation should fail with errno set to ENOTSOCK

• [X] Client programs should be simplified to take into account this new behaviour.

[mfragkoulis]

The only concern with client programs is with the way dgsh-wrap handles arguments, which will be addressed with the fix for issue #53 .