nspawn: Simplify mkdir_userns() usage, and trickle that up

nspawn: Simplify mkdir_userns() usage, and trickle that up

One of the things that mkdir_userns{,_p}() does is take an (optional) UID,
and chown the directory to that.  So we need a uid_t argument, and a way of
telling if we should use that uid_t argument.  Fortunately, that is built
in to the uid_t type by having UID_INVALID as a possible value.

However, currently mkdir_userns() also takes a MountSettingsMask and checks
a couple of bits in it to decide if it should perform the chown.

Drop the mask argument, and instead have the caller pass UID_INVALID if it
shouldn't chown.