initdb: introduce PG_CMD_PRINTF intestead of PG_CMD_PRINTF{1,2,3}

Hello all,

The src/bin/initdb/initdb.c provides three macros to write data to
cmdfd. All of these macro do the same, but with different amount of
arguments for fprintf().

Attached patch introduces PG_CMD_PRINTF macro which will take set of
variadic arguments via __VA_ARGS__ to replace the PG_CMD_PRINTF{1,2,3}
macros.

Any objections?