lib: Preserve errno in our malloc() and free() wrappers

lib: Preserve errno in our malloc() and free() wrappers

Various places assume that e.g. t_strdup_printf() calls and such don't
modify errno. But because they internally call malloc() or calloc(), this
isn't actually guaranteed now and it can happen at least with newer glibc
versions. Explicitly preserve the errno for these calls where it might
be a problem.