lib/strutils: fix string_add_to_idarray() int vs. size_t

lib/strutils: fix string_add_to_idarray() int vs. size_t

The function uses "int" as argument, but for array size (and index) is better
to use unsigned type (size_t). If we mix "size_t" in util (e.g. fdisk)
and "int" in lib/strutils.c then result is unexpected behavior on
ppc64.

	# sfdisk --list -o DEVICE,START,SIZE /dev/sdb
	Disk /dev/sdb: 50 MiB, 52428800 bytes, 102400 sectors
	Units: sectors of 1 * 512 = 512 bytes
	Sector size (logical/physical): 512 bytes / 4096 bytes
	I/O size (minimum/optimal): 4096 bytes / 32768 bytes
	Disklabel type: gpt
	Disk identifier: 3B8559DB-33AF-43E9-BEFC-C331D829B539
	lt-sfdisk: libfdisk/src/label.c:178: fdisk_label_get_field: Assertion `id > 0' failed.

The patch cleanup all code to use size_t everywhere.

Signed-off-by: Karel Zak <kzak@redhat.com>