fionread, used in poll_oneoff on Unix, may overflow for large files

[marmistrz]

ioctl_tty(2) says that fionread accepts an int*, which may overflow for large files.
Incidentally, the correct size will be returned if we pass a pointer to int64_t, but this something we probably shouldn't rely on.

We probably should use a different syscall (fstat?) to query file size for regular files instead of fionread. Relevant discussion occurs in #552.
See this example C code which exhibits this behavior.

#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <stdint.h>

int main(int argc, char** argv) {
	if (argc != 2) {
		fprintf(stderr, "Usage: %s [file path]\n", argv[0]);
		exit(1);
	}
	FILE* file = fopen(argv[1], "r");
	if (file == NULL) {
		perror("opening file");
		exit(1);
	}
	int fd = fileno(file);
	int avail = 0;
	//int64_t avail = 0;
	int ret = ioctl(fd, FIONREAD, &avail);
	if (ret == -1) {
		perror("ioctl");
		exit(1);
	}

	printf("Bytes available: %d\n", avail);
	//printf("Bytes available: %ld\n", avail);
	return 0;
}