glyph_to_cmap() becomes function instead of macro

Also adds swallow and explosion glyphs to glyph_to_cmap()

References:
https://github.com/NetHack/NetHack/pull/1022 by argrath
https://github.com/NetHack/NetHack/pull/1277 by ars3niy

Closes #1277
Closes #1022

src/glyphs.c

@@ -221,7 +221,8 @@ glyph_to_cmap(int glyph)
     else if (glyph_is_explosion(glyph))
         return glyph_to_explosion(glyph) + S_expl_tl;
     else
-        return MAXPCHARS;
+        return MAXPCHARS;    /* MAXPCHARS is legal array index because
+                                of trailing fencepost entry */
 }
 
 staticfn int