pkt-line: add stdio packet write functions

pkt-line: add stdio packet write functions

This adds three new functions to pkt-line.c: packet_fwrite,
packet_fwrite_fmt and packet_fflush. Besides writing a pktline flush
packet, packet_fflush also flushes the stdio buffer of the stream.

Helped-by: Patrick Steinhardt <redacted>
Helped-by: Jeff King <redacted>
Signed-off-by: Jacob Vosmaer <redacted>
Signed-off-by: Junio C Hamano <redacted>