xzgrep: Add more LC_ALL=C to avoid bugs with multibyte characters.

Also replace one use of expr with printf.

The rationale for LC_ALL=C was already mentioned in
69d1b3fc29677af8ade8dc15dba83f0589cb63d6 that fixed a security
issue. However, unrelated uses weren't changed in that commit yet.

POSIX says that with sed and such tools one should use LC_ALL=C
to ensure predictable behavior when strings contain byte sequences
that aren't valid multibyte characters in the current locale. See
under "Application usage" in here:

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/sed.html

With GNU sed invalid multibyte strings would work without this;
it's documented in its Texinfo manual. Some other implementations
aren't so forgiving.

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