Use an mmap()'d buffer instead of a static buffer for the contents

Use an mmap()'d buffer instead of a static buffer for the contents
of the pointer returned by getpw{ent,nam,uid}().  We unmap the
buffer each time to catch callers using a stale passwd struct
pointer.  As a special case, we do not unmap the buffer if the
previous lookup was for the same name or uid.  This special case
may be removed in the future.  OK deraadt@