locksrv/ls: Make -p optional, and validate it if it exists
locksrv/ls: Make -p optional, and validate it if it exists

- If -p is provided on the command line, function as we did before (except
see below).  If -p is not provided, list every parcel on the server.

- No longer assume that the parcel specified by -p actually exists.  If it
does not exist, print a message and return nonzero exit status.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr