Windows: Fix signal numbers

We had defined some SIG_FOO macros that appear in the code, but that are
not supported on Windows, in order to make the code compile.  But a
subsequent change will assert that a signal number is non-zero.  We now
use the signal numbers that are commonly used on POSIX systems.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -21,12 +21,12 @@ typedef int pid_t;
 #define WEXITSTATUS(x) ((x) & 0xff)
 #define WIFSIGNALED(x) ((unsigned)(x) > 259)
 
-#define SIGKILL 0
-#define SIGCHLD 0
-#define SIGPIPE 0
-#define SIGHUP 0
-#define SIGQUIT 0
-#define SIGALRM 100
+#define SIGHUP 1
+#define SIGQUIT 3
+#define SIGKILL 9
+#define SIGPIPE 13
+#define SIGALRM 14
+#define SIGCHLD 17
 
 #define F_GETFD 1
 #define F_SETFD 2