Refactor getobj() to use callbacks on candidate objects

This replaces the arcane system previously used by getobj where the
caller would pass in a "string" whose characters were object class
numbers, with the first up to four characters being special constants
that effectively acted as flags and had to be in a certain order.
Because there are many places where getobj must behave more granularly
than just object class filtering, this was supplemented by over a
hundred lines enumerating all these special cases and "ugly checks", as
well as other ugly code spread around in getobj callers that formatted
the "string".

Now, getobj callers pass in a callback which will return one of five
possible values for any given object in the player's inventory. The
logic of determining the eligibility of a given object is handled in the
caller, which greatly simplifies the code and makes it clearer to read.
Particularly since there's no real need to cram everything into one if
statement.

This is related to pull request #77 by FIQ; it's largely a
reimplementation of its callbacks system, without doing a bigger than
necessary refactor of getobj or adding the ability to select a
floor/trap/dungeon feature with getobj. Differences in implementation
are mostly minor:
- using enum constants for returns instead of magic numbers
- 5 possible return values for callbacks instead of 3, due to trying to
  make it behave exactly as it did previously. PR #77 would sometimes
  outright exclude objects because it lacked semantics for invalid
  objects that should be selectable anyway, or give slightly different
  messages.
- passing a bitmask of flags to getobj rather than booleans (easier to
  add more flags later - such as FIQ's "allow floor features" flag, if
  that becomes desirable)
- renaming some of getobj's variables to clearer versions
- naming all callbacks consistently with "_ok"
- generally more comments explaining things

The callbacks use the same logic from getobj_obj_exclude,
getobj_obj_exclude_too and getobj_obj_acceptable_unlisted (and in a few
cases, from special cases still within getobj). In a number of them, I
added comments suggesting possible further refinements to what is and
isn't eligible (e.g. should a bullwhip really be presented as a
candidate for readying a thrown weapon?)

This also removed ALLOW_COUNT and ALLOW_NONE, relics of the old system,
and moved ALLOW_ALL's definition into detect.c which is the only place
it's used now (unrelated to getobj). The ALLOW_ALL functionality still
exists as the GETOBJ_PROMPT flag, because its main use is to force
getobj to prompt for input even if nothing is valid.

I did not refactor ggetobj() as part of this change.

src/engrave.c

@@ -5,6 +5,7 @@
 
 #include "hack.h"
 
+static int FDECL(stylus_ok, (struct obj *));
 static const char *NDECL(blengr);
 
 char *
@@ -435,10 +436,28 @@ freehand()
             || (!bimanual(uwep) && (!uarms || !uarms->cursed)));
 }
 
-static NEARDATA const char styluses[] = { ALL_CLASSES, ALLOW_NONE,
-                                          TOOL_CLASS,  WEAPON_CLASS,
-                                          WAND_CLASS,  GEM_CLASS,
-                                          RING_CLASS,  0 };
+/* getobj callback for an object to engrave with */
+static int
+stylus_ok(obj)
+struct obj *obj;
+{
+    if (!obj)
+        return GETOBJ_SUGGEST;
+
+    /* Potential extension: exclude weapons that don't make any sense (such as
+     * bullwhips) and downplay rings and gems that wouldn't be good to write
+     * with (such as glass and non-gem rings) */
+    if (obj->oclass == WEAPON_CLASS || obj->oclass == WAND_CLASS
+        || obj->oclass == GEM_CLASS || obj->oclass == RING_CLASS)
+        return GETOBJ_SUGGEST;
+
+    /* Only markers and towels are recommended tools. */
+    if (obj->oclass == TOOL_CLASS
+        && (obj->otyp == TOWEL || obj->otyp == MAGIC_MARKER))
+        return GETOBJ_SUGGEST;
+
+    return GETOBJ_EXCLUDE;
+}
 
 /* Mohs' Hardness Scale:
  *  1 - Talc             6 - Orthoclase
@@ -545,7 +564,7 @@ doengrave()
      * Edited by GAN 10/20/86 so as not to change weapon wielded.
      */
 
-    otmp = getobj(styluses, "write with");
+    otmp = getobj("write with", stylus_ok, GETOBJ_PROMPT);
     if (!otmp) /* otmp == cg.zeroobj if fingers */
         return 0;