[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. 

Me too. Could be a small inconsistency. But haven't really looked into it.

> Should we just change the signature of os_fchown to accept uint64_t?

Could be an option.

> Also, uv_uid_t resolves to unsigned int. Shouldn't we change -1 to UINT_MAX or is (unsigned int)-1 an acceptable C idiom?

The fact that uv_uid_t resolves to unsigned int should remain hidden, so that it can be changed. So I don't like relying on UINT_MAX. The ideal thing should be having a constant UV_UID_MAX. Lacking that, I don't think the (uv_uid_t)-1 idiom is bad, as that would automatically adapt to changes.