group on non-unix like systems [was: ls-lisp and remote files]

[Lars Hansen]

 Yesterday I installed changes that add a new optional parameter 
id-format to file-attributes and directory-files-and-attributes. Today I 
realize that those changes don't compile on windows and mac because 
these systems don't have the getgrgid function. To fix that, I have 
implemented the missing function in src/w32.c and src/mac.c and defined 
struct group in new files nt/inc/grp.h and mac/inc/grp.h. The function 
getgrgid is just a stub that always returns "root" as group name. Maybe 
the people that maintain windows and mac ports want to check my changes. 
If so, thank you for your help!

By the way, the files nt/inc/grp.h and mac/inc/grp.h are new, so they 
need tags. How do one create them?