replace getenv("TTYINV") with perminv_mode option

Add a new option 'perminv_mode' to augment perm_invent.  It handles
the same choices as the temporary TTYINV method:  show all items other
than gold, show full inventory including gold, or only show in-use
items (similar to the '*' command).

For tty, both the all-except-gold and full-inventory modes can add
the poorly named 'sparse' variation which populates unused slots in
its fixed grid with the inventory letter that would go in each.

For others, the default has been changed from full-inventory to
all-except-gold.  Note that gold is treated as part of 'all' or of
'in-use' if it is quivered because having the amount be shown on the
status line doesn't make that redundant.

Changing the default may mess up WinGUI if it assumes that perm_invent
is full inventory with gold.

Initially I was going to change perm_invent into a compound but this
leaves it as an on/off toggle and adds perminv_mode as a separate
option for how to show the inventory when the toggle is on.  It may
make sense to combine them since dual controls is a little confusing,
but right now setting perm_invent On when perminv_mode is 'none'
changes that to 'all' and changing perminv_mode away from 'none' when
perm_invent is Off toggles it to On.

Guidebook.mn has been updated but as usual Guidebook.tex is lagging.

dat/opthelp

@@ -231,6 +231,15 @@ paranoid_confirmation  space separated list    [paranoid_confirm:pray swim]
                       in the object class filtering menu is selected;
               Remove  -- always pick from inventory for 'R' and 'T' even when
                       wearing just one applicable item to remove or take off
+perminv_mode  if the interface supports a persistent inventory window   [a]
+              and the perm_invent option is true, this controls what
+              will be shown:
+              none/off -- behave as if perm_invent is false
+              all/on   -- show inventory except for gold (default)
+              full     -- show inventory including gold
+              in-use   -- only show worn and wielded items
+              (the tty interface's optional perm_invent support includes
+              a couple of additional choices that vary all and full)
 pickup_burden when you pick up an item that exceeds this encumbrance    [S]
               level (Unencumbered, Burdened, streSsed, straiNed, overTaxed,
               or overLoaded), you will be asked if you want to continue.