Add functions to get system user names, group names

Add functions to get system user names, group names

Note from committer:
I removed the part that adds grp.h to AC_CHECK_HEADERS and

+#ifdef HAVE_GRP_H
 #include <grp.h>
+#endif

to src/dired.c, because the latter has unconditionally included grp.h
since 2003, and uses it eg in stat_gname.

* configure.in (AC_CHECK_FUNCS): Add getpwent, endpwent, getgrent, endgrent.

* src/dired.c (Fsystem_users, Fsystem_groups): New functions.
(syms_of_dired): Add them.