Fails to build on architectures with unsigned c char.

[plugwash]

After updating condure in Debian to 1.10, it failed to build on architectures where char in c is unsigned (e.g. arm*, powerpc*, s390x). The fix was pretty trivial, just replacing i8 with libc::c_char in a couple of pointer typecasts.

Patch is available at https://salsa.debian.org/rust-team/debcargo-conf/-/blob/171ebbf31695774186bd7d279031906c7f179082/src/condure/debian/patches/fix-build-unsigned-char.diff