[RFC] Remove project-specific integer types: long_u. (3)

Continues #1340.

Removes 57 `long_u` instances.
104 still remaining for future PR's.
I wonder why libuv's own `uv_stat_t.st_gid` is defined as `uint64_t`. Should we just change the signature of `os_fchown` to accept `uint64_t`? cc @stefan991