New API calls: truncate, truncate_size, mkdir_mode, utimens, lchown.

New API calls: truncate, truncate_size, mkdir_mode, utimens, lchown.

truncate, truncate_size: Used to truncate files to a particular
size, or to zero bytes.

mkdir_mode: Like mkdir but allows you to also specify the
initial permissions for the new directory.

utimens: Set timestamp on a file with nanosecond accuracy.

lchown: Corresponding to lchown(2) syscall (we already have chown).

The implementation is complicated by the fact that we had to
add an Int64 parameter type to the generator.