Merge branch 'jk/fallthrough'

Many codepaths have been updated to squelch -Wimplicit-fallthrough
warnings from Gcc 7 (which is a good code hygiene).

* jk/fallthrough:
  consistently use "fallthrough" comments in switches
  curl_trace(): eliminate switch fallthrough
  test-line-buffer: simplify command parsing

config.c

@@ -2355,6 +2355,7 @@ static ssize_t write_pair(int fd, const char *key, const char *value)
 		case '"':
 		case '\\':
 			strbuf_addch(&sb, '\\');
+			/* fallthrough */
 		default:
 			strbuf_addch(&sb, value[i]);
 			break;