color.h: document and modernize header

Add documentation explaining the functions in color.h.
While at it, migrate the function `color_set` into grep.c,
where the only callers are.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -18,6 +18,11 @@ static void std_output(struct grep_opt *opt, const void *buf, size_t size)
 	fwrite(buf, size, 1, stdout);
 }
 
+static void color_set(char *dst, const char *color_bytes)
+{
+	xsnprintf(dst, COLOR_MAXLEN, "%s", color_bytes);
+}
+
 /*
  * Initialize the grep_defaults template with hardcoded defaults.
  * We could let the compiler do this, but without C99 initializers