retreat lretreat: Change query syntax to match mission

retreat lretreat: Change query syntax to match mission

Optional arguments can save typing.  Mandatory arguments are more
easily discoverable: just run the command and answer its prompts.
Empire traditionally uses optional arguments only for expert features.
Consider mission:

    [0:640] Command : mission
    Ship, plane or land unit (p,sh,la)? s
    ship(s)? 0
    Mission (int, sup, osup, dsup, esc, res, air, query, clear)? int
    operations point? .
    frg  frigate Early Bird(#0) on an interdiction mission, centered on 21,-3, radius 0
    1 ship

Compare retreat:

    [0:638] Command : retreat
    ship(s)? 0
    shp#     ship type       x,y   fl path       as flt?  flags
       0 frg  frigate       21,-3
    1 ship

Arguments are not discoverable this way.

Change retreat to work like mission: make the second argument
mandatory, and if it's 'q', show retreat orders, else treat it as path
and ask for conditions:

    [0:637] Command : retreat
    ship(s)? 0
    Retreat path, or q to query? jj
    Retreat conditions ('?' to list available ones)? i
    shp#     ship type       x,y   fl path       as flt?  flags
       0 frg  frigate       21,-3     jj                  i
    1 ship

To reduce smart client and script breakage, keep retreat with one
argument working as before, but print a deprecation warning.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>