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

include/extern.h

@@ -1124,6 +1124,7 @@ extern void purge_all_custom_entries(void);
 extern void dump_glyphids(void);
 extern void clear_all_glyphmap_colors(void);
 extern void reset_customcolors(void);
+extern int glyph_to_cmap(int);
 
 /* ### hack.c ### */