When system calls indicate an error they return -1, not some arbitrary

value < 0.  errno is only updated in this case.  Change all (most?)
callers of syscalls to follow this better, and let's see if this strictness
helps us in the future.

client.c

@@ -434,7 +434,7 @@ client_stdin_callback(__unused int fd, __unused short events,
 	struct msg_stdin_data	data;
 
 	data.size = read(STDIN_FILENO, data.data, sizeof data.data);
-	if (data.size < 0 && (errno == EINTR || errno == EAGAIN))
+	if (data.size == -1 && (errno == EINTR || errno == EAGAIN))
 		return;
 
 	proc_send(client_peer, MSG_STDIN, -1, &data, sizeof data);

job.c

@@ -118,7 +118,7 @@ job_run(const char *cmd, struct session *s, const char *cwd,
 		close(out[0]);
 
 		nullfd = open(_PATH_DEVNULL, O_RDWR, 0);
-		if (nullfd < 0)
+		if (nullfd == -1)
 			fatal("open failed");
 		if (dup2(nullfd, STDERR_FILENO) == -1)
 			fatal("dup2 failed");