New siginfo member in uev_t for signal watchers

This patch adds support for exporting the signalfd_siginfo struct read
from the signal fd descriptor on signals.  This allows watcher callbacks
to, e.g., figure out who sent a signal.  See signalfd(2) for details on
struct signalfd_siginfo.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/uev.h

@@ -61,6 +61,9 @@ typedef struct uev {
 	int             signo;
 	int             fd;
 	uev_ctx_t      *ctx;
+
+	/* Extra data for certain watcher types */
+	struct signalfd_siginfo siginfo;
 } uev_t;
 
 /*