Commit sid<->[ug]id extensions to libnss_winbind.so on Linux, as well as a

Commit sid<->[ug]id extensions to libnss_winbind.so on Linux, as well as a
factored-out nss_winbind.[ch]. I took tridge's public domain license comment
for the nss_winbind.[ch].

This is probably not the last word on that extension, but as it is quite
non-intrusive to the main samba code, I would like to give it a start.

Volker