basic/glyph-util: rename "special glyph" to just "glyph"

Admittedly, some of our glyphs _are_ special, e.g. "O=" for SPECIAL_GLYPH_TOUCH ;)
But we don't need this in the name. The very long names make some invocations
very wordy, e.g. special_glyph(SPECIAL_GLYPH_SLIGHTLY_UNHAPPY_SMILEY).
Also, I want to add GLYPH_SPACE, which is not special at all.

src/basic/string-util.c

@@ -247,7 +247,7 @@ bool string_has_cc(const char *p, const char *ok) {
 }
 
 static int write_ellipsis(char *buf, bool unicode) {
-        const char *s = special_glyph_full(SPECIAL_GLYPH_ELLIPSIS, unicode);
+        const char *s = glyph_full(GLYPH_ELLIPSIS, unicode);
         assert(strlen(s) == 3);
         memcpy(buf, s, 3);
         return 3;