Rename struct linebufline to struct line and add linecmp()

This simplifies the handling in sort(1) and comm(1) quite a bit.

text.h

@@ -1,12 +1,12 @@
 /* See LICENSE file for copyright and license details. */
 
-struct linebufline {
+struct line {
 	char *data;
 	size_t len;
 };
 
 struct linebuf {
-	struct linebufline *lines;
+	struct line *lines;
 	size_t nlines;
 	size_t capacity;
 };
@@ -14,3 +14,4 @@ struct linebuf {
 void getlines(FILE *, struct linebuf *);
 
 void concat(FILE *, const char *, FILE *, const char *);
+int linecmp(struct line *, struct line *);