[linebuf] Support buffering of multiple lines

Allow line buffer to accumulate multiple lines, with buffered_line()
returning each freshly-completed line as it is encountered.  This
allows buffered lines to be subsequently processed as a group.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/tests/tests.c

@@ -66,3 +66,4 @@ REQUIRE_OBJECT ( uri_test );
 REQUIRE_OBJECT ( profile_test );
 REQUIRE_OBJECT ( setjmp_test );
 REQUIRE_OBJECT ( pccrc_test );
+REQUIRE_OBJECT ( linebuf_test );