initdb: Use varargs macro for PG_CMD_PRINTF

initdb: Use varargs macro for PG_CMD_PRINTF

I left PG_CMD_PUTS around even though it could be handled by
PG_CMD_PRINTF since PG_CMD_PUTS is sometimes called with non-literal
arguments, and so that would create a potential problem if such a
string contained percent signs.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>