hooks/fsck: Fix invocation of fstype at build time

get_fstab prefers to use the fstype utility (from klibc) and then
falls back to blkid.  Since klibc utilities are not in $PATH at build
time, get_fsck_type_fstab defines fstype as an alias for its full
filename before calling get_fstab.

This never actually worked, because aliases are expanded during
function definition, not execution.

Replacing the alias and unalias with a shell function definition and
an 'unset -f' solves the problem.

Thanks: Simon Tatham <anakin@pobox.com>
Closes: #865691
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

hooks/fsck

@@ -59,9 +59,9 @@ get_fsck_type_fstab () {
 		# therefore never used at boot time
 		if [ "${MNT_DIR}" = "/" ] || [ "${MNT_TYPE}" = "auto" ]; then
 			MNT_FSNAME="$(resolve_device "${MNT_FSNAME}")"
-			alias fstype="/usr/lib/klibc/bin/fstype"
+			fstype() { "/usr/lib/klibc/bin/fstype" "$@"; }
 			get_fstype "${MNT_FSNAME}"
-			unalias fstype
+			unset -f fstype
 		else
 			echo "${MNT_TYPE}"
 		fi