Fix grub-emu build on FreeBSD.

Fix grub-emu build on FreeBSD.

* Makefile.util.def (grub-mount): Add LIBGEOM to ldadd.
* grub-core/net/drivers/emu/emunet.c: Only include Linux-specific
headers on Linux.
(GRUB_MOD_INIT): Return immediately on non-Linux platforms; this
implementation is currently Linux-specific.
* util/getroot.c (exec_pipe): Define only on Linux or when either
libzfs or libnvpair is unavailable.
(find_root_devices_from_poolname): Remove unused path variable.