Refactor: libcrmcommon: Avoid snprintf() in time_as_string_common()

We don't error-check the return code, and the offset update idiom is
painful to look at. It won't hurt us to do a tiny bit more string
allocation in the heap for readability. We already do a ton of
allocation for XML and Pacemaker data structures.

Ref T974

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>