OpenBSD fallocate

Hi,

OpenBSD doesn't have either fallocate or posix_fallocate but it does have ftruncate:

int ftruncate(int fd, off_t length);

OpenBSD's renameat also appears to be different than Linux and FreeBSD:

int renameat(int fromfd, const char *from, int tofd, const char *to);

I'm happy to help in any way I can to get go-fuse running on OpenBSD although I don't know Go, but I am ready to learn!

