Add "level up" functionality, etc.

Added "level up" functionality to "damage_npc" (i.e., when an NPC dies
and grants experience points), adjusted "level up" narration text, added
an "item" data member to the "npc_t" struct, adjusted "damage_npc" to
have items dropped as loot when NPCs die, increased the use of
"menu_layer_set_selected_index" in "set_game_mode", fixed some run-time
errors and made cosmetic adjustments in "menu_draw_row_callback",
"menu_select_callback", and "init_location", adjusted "init_npc" to make
NPCs weaker, made minor adjustments to several other functions, and
renamed the DEFAULT_BASE_STAT_VALUE as DEFAULT_MAJOR_STAT_VALUE while
increasing it to 3.