Various issues with plural handling

[myshkin]

Objects and monsters should have the same plural handling code. Monster pluralization should move to the edit file. While we're at it, fix Mumak/Mumakil and Craban/Crebain plurals.

[magnate]

Punting in accordance with new milestone policy (that any other milestone is only set once someone is actually working on the ticket).

[magnate]

There's also a pluraliser macro in monster1.c (and probably others - needs a grep for "plural").

[myshkin]

Other plural issues:

• h-basic.h defines PLURAL(n), while monster/mon-lore.c defines plural(c, s, p). I assume the latter is the macro to which magnate refers.
• object/obj-util.c and wiz-stats.c call PLURAL(plural) to get an "s" or lack thereof for verb agreement. Here, plural is a bool. Since the PLURAL macro tests its argument against 1, that the macro works is quite difficult to see.
• monster/mon-lore.c fails to do the same convoluted dance with the "who remain%s unavenged" string in describe_monster_kills(), and so our grammar is wrong. Spotted by Derakon on ​oook.
• spells1.c:project_o() eschews these macros and rolls its own plural-handling code.
• cmd-obj.c:activation_message() also has some verb agreement code.
• The borg has some code for plurals as well.