v0.6.20

fix: send the NGT-1 "receive all PGNs" command — no more heartbeats-only

By default the Actisense NGT-1 forwards only network-management PGNs (126993
heartbeat, 60928 address claim); sensor data (engine, GPS, rudder, …) never
reaches the host until it is told to emit everything. We never sent that
command, so on a freshly power-cycled NGT-1 pgntui saw only heartbeats. (It
appeared to work earlier only because a prior NMEA Reader session had left the
gateway in receive-all mode.)

Send the canboat NGT_STARTUP_SEQ — NGT_MSG_SEND (0xA1) with payload
0x11 0x02 0x00, which clears the gateway's PGN TX filter — on open(), and
re-send it every 20s in read_frames() to keep the mode active (matching
canboat actisense-serial). Bytes verified against canboat actisense.h /
actisense-serial.c.

Bumps to 0.6.20.