Solaris support in grub_find_zpool_from_dir(). Thanks
Seth Goldberg for referring to getextmntent() facility.
* configure.ac: Check for getextmntent(), `sys/mnttab.h' and
`sys/mkdev.h'.
* grub-core/kern/emu/misc.c [HAVE_SYS_MNTTAB_H]: Include
`<sys/mnttab.h>'.
[HAVE_SYS_MKDEV_H]: Include `<sys/mkdev.h>'.
[HAVE_GETEXTMNTENT] (grub_find_zpool_from_dir): Add getextmntent()
method for finding zpool name.