search: remember last searched-for string between searches

Regardless of how we exit search mode (commit or cancel), the search
string is remembered.

The next time we enter search mode, the last searched-for string will
be used when searching for the next/prev match (ctrl+r, ctrl+s), and
the search query is empty.

terminal.h

@@ -514,6 +514,11 @@ struct terminal {
         bool view_followed_offset;
         struct coord match;
         size_t match_len;
+
+        struct {
+            wchar_t *buf;
+            size_t len;
+        } last;
     } search;
 
     struct wayland *wl;