Use malloc()ed buffers instead of stack buffers in gr_copy() and pw_copy().

Use malloc()ed buffers instead of stack buffers in gr_copy() and pw_copy().
This allows pw(8) to operate on passwd and group files with longer lines
than could be accomodated by a stack buffer.  It doesn't take more than a
few hundred users to exceed 8192 bytes in /etc/group.

MFC after:	3 weeks
Sponsored by:	The University of Oslo