add grub_qsort_strcmp to use when sorting array of strings

add grub_qsort_strcmp to use when sorting array of strings

Compare function used in qsort gets arguments by reference, so strcmp
cannot be used directly - it expects pointer to char, but gets pointer
to pointer to char.

Introduce new helper grub_qsort_strcmp and use it in grub-install.
This helper is going to be used in a couple more places as well so
add it to global file, not in grub-install.c.