Correct Map for file sizes larger than 1<<31 - 1

int(int64value) doesn't always return <0 when an int is 32 bits.
Long ago, Go moved to 64 bit int's on amd64 and mmap-ing large files works fine.
Should not panic when an error return is the sane thing to do.
Remove un-used ANON (which should probably have been set to syscall.MAP_ANON anyway).