pkt-line: add functions to read/write flush terminated packet streams

pkt-line: add functions to read/write flush terminated packet streams

write_packetized_from_fd() and write_packetized_from_buf() write a
stream of packets. All content packets use the maximal packet size
except for the last one. After the last content packet a `flush` control
packet is written.

read_packetized_to_strbuf() reads arbitrary sized packets until it
detects a `flush` packet.

Signed-off-by: Lars Schneider <redacted>
Signed-off-by: Junio C Hamano <redacted>