__time_to_tm: initialize tm_zone and tm_gmtoff
make tm_zone etc visible under _GNU_SOURCE
add arpa/nameser_compat.h

the contents of this header are already in arpa/nameser.h
re-add useconds_t

this type was removed back in 5243e5f1606a9c6fcf01414e ,
because it was removed from the XSI specs.
however some apps use it.
since it's in the POSIX reserved namespace, we can expose it
unconditionally.