input: use a timer fd to handle keyboard key repeat

Instead of running a repeater thread that writes the key to repeat
over a pipe, use a simple timer fd.

No more locking or condition signalling. No need to track
start/stop/exist states.

We simply set up the initial timeout value to be the 'delay', and the
interval to be the repeat 'rate'.

terminal.h

@@ -156,12 +156,7 @@ struct kbd {
     struct xkb_compose_table *xkb_compose_table;
     struct xkb_compose_state *xkb_compose_state;
     struct {
-        mtx_t mutex;
-        cnd_t cond;
-        int trigger;
-        int pipe_read_fd;
-        int pipe_write_fd;
-        enum {REPEAT_STOP, REPEAT_START, REPEAT_EXIT} cmd;
+        int fd;
 
         bool dont_re_repeat;
         int32_t delay;