macro C_WRITE_NOW. Updated all callers.

Rev: src/client_keyexchange.c:1.65
Rev: src/connection.c:1.79
Rev: src/debug.c:1.39
Rev: src/gateway_commands.c:1.21
Rev: src/handshake.c:1.20
Rev: src/keyexchange.c:1.91
Rev: src/lsh-decode-key.c:1.21
Rev: src/lshg.c:1.23
Rev: src/read_file.c:1.7
Rev: src/server.c:1.76
Rev: src/server_keyexchange.c:1.56
Rev: src/unix_user.c:1.64