mac-types:  Add VRefNum_DirID structure type

I wasn't sure what to call this.  Nitrogen has the name FSDirSpec, which
is reasonable enough but doesn't convey that it's a third-party addition
rather an original member of the namespace.  Rather than inject the most
Toolboxish name I can think of as an intended equal among its neighbors,
I've opted for austerity.  The name VRefNum_DirID tells you exactly what
the struct contains and nothing more.  (And the underscore is a giveaway
that it's not part of the Toolbox.)
mac-sys-utils:  Add get_user_home()
Genie:  Use mac::sys::get_user_home()
Merge branch 'get_user_home'
mac-types:  Add MultipleUsersState

This is the structure defined (and poorly named "GestaltRec") in Apple's
tech note #1186.[1]

[1] Technical Note TN1186: "How To Be a Good Multiple Users Citizen".
mac-sys-utils:  Support At Ease in get_user_home()