Refactor: libcrmcommon: Error-check snprintf() in pcmk_readable_score()

Also avoid unnecessary strcpy() calls by returning the desired string
without copying it to a buffer.

The xstr() and str() macros are unnecessary if we just hard-code
sizeof("-1000000"). I try to avoid using magic numbers but I don't have
strong feelings.

Ref T974

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