Instead of keeping separate groups and gids arrays, create struct

group_info and use it to store both, along with a count for each.
Cache group info on a per-user basis using getgrouplist() to get
the groups.  We no longer need special to special case the user or
list user for user_in_group() and thus no longer need to reset the
groups list when listing another user.

include/missing.h

@@ -286,6 +286,9 @@ void closefrom(int);
 #ifndef HAVE_GETCWD
 char *getcwd(char *, size_t size);
 #endif
+#ifndef HAVE_GETGROUPLIST
+int getgrouplist(const char *, gid_t, gid_t *, int *);
+#endif
 #ifndef HAVE_GETLINE
 ssize_t getline(char **, size_t *, FILE *);
 #endif