fs: uids and gids must be unsigned ints

Before this commit, fs.chown() and fs.fchown() coerced the uid and gid
arguments to signed integers which is wrong because uid_t and gid_t are
unsigned on most all platforms and IDs that don't fit in a signed
integer do exist.

This commit changes the aforementioned functions to take unsigned ints
instead.  No test because we can't assume the system has [GU]IDs that
large.

This change depends on joyent/libuv@d779eb5.

Fixes #5890.