Make retr() and lretr() simpler and more robust

Make retr() and lretr() simpler and more robust

The old code recognized group retreat only when the first argument was
on the command line.  Didn't make a difference, because it was only
used when there were at least two arguments on the command line.

The old code relied on rflags being represented as two's complement.

When given an empty retreat path, the old code deleted the retreat
path and set the retreat flags normally.  The new code deletes both.
Neither is nice; it should perhaps keep the retreat path and only set
the flags.