Move signal handling to libuv event loop

This removes all signal handling code from os_unix.c to os/signal.c. Now signal
handling is done like this:

- Watchers for signals are registered with libuv default event loop
- `event_poll` continuously calls `poll_uv_loop` to produce events until it
  receives user input, SIGINT or a timeout
- Any signals received in `poll_uv_loop` will push events to a queue that is
  drained and processed by `event_poll`

Signals aren't handled directly in the libuv callback to avoid recursion in the
event loop(which isn't supported by libuv).

The same principle will apply to other events in the future: Push to a queue
from a libuv callback and drain it from `event_poll`

src/os_unix.h

@@ -5,9 +5,6 @@
 
 /* os_unix.c */
 void mch_write(char_u *s, int len);
-void mch_startjmp(void);
-void mch_endjmp(void);
-void mch_didjmp(void);
 void mch_suspend(void);
 void mch_init(void);
 void reset_signals(void);