* Hostgroup stack

Rex now maintains a stack of hostgroups.  Once you push a hostgroup
name on stack, that hostgroup will be used by any subsequent rex
command, unless it is given the -g option.  The new command "group"
(see below) provides ways to pop items off the stack, swap arbitrary
element with the top of stack, select new hostgroups on the fly, etc.

* New command: rex group

The "rex group" command has the following forms:

  rex group push GROUP
    Push GROUP on stack.

  rex group pop
    Pop the topmost group off the stack.

  rex group swap N
    Exchange top of the stack with the Nth element (0-based).

  rex group drop N
    Remove Nth element from the stack.

  rex group select COMMAND ARGS...
    Create on top of the stack a temporary group that contains
    those hosts from the current hostgroup where the supplied
    shell command returns success, i.e. exits with code 0.

  rex group show
    List the contents of the hostgroup stack.

* Add configuration function for overriding PTR records from rc files