Don't PROC_DUP_FGRP, instead SYS_DUP_FDS_TO

Previously we were using the PROC_DUP_FGRP flag to the proc_create()
syscall to, telling akaros to straight up dup the fds of a parent into
the same fd numbers in the child.  We actually want to do a remapping
though, which the new SYS_DUP_FDS_TO syscall allows us to do.