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

NAME_MAX isn't uniformly enforced (see URL below), and this avoids the
need to explicitly error-check snprintf().

www.gnu.org/software/libc/manual/html_node/Limits-for-Files.html

Ref T974

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