Fix some issues raised (albeit indirectly) by Isaac Dunham.

POLL_IN defined as a constant by some libc.
Factor out login.c's change_identity() to xwrap.c as xsetuser().
Replace xsetuid() with xsetuser()
Put a space between argument globals and non-argument globals.
TT starts zeroed, don't need to re-zero entries in it.
STDIN_FILENO has been 0 since 1969, even DOS copied that. Just say 0.
Added an xchroot() using xchdir() to lib/xwrap.c.
Remove endgrent() call until somebody can explain why it was there.
IPv6 support is added into traceroute command.
traceroute6 is made as an old toy of traceroute.
Pending commands default n.
Attached is an implementation for groupdel.

This is aliased to delgroup, for a requirement at my end.
Also lib/password.c is modified to along with.
Updating the email id in Copyright.
Clean up half-finished blkid.test.