pos2nmea: Enforce line buffering

When connected to stdout, perl uses line buffering. If not connected to
stdout (e.g. from a pipe) it defaults to block buffering. This causes a
long delay and decoding issues when using the NMEA output in other
tools.

With this change always use line buffering so that NMEA messages are
flushed immediately.

Tested with NMEAoIP and GPSD:

<decoder> | pos2nmea.pl | socat - tcp-l:4444,reuseaddr,fork
<decoder> | pos2nmea.pl | gpsd -Nnrb /dev/stdin

tools/pos2nmea.pl

@@ -51,6 +51,8 @@ else {
 
 my $geoid = 50.0;  ## GPS ueber Ellipsoid; Geoid-Hoehe in Europa ca 40-50m
 
+$| = 1;  ## line buffering erzwingen
+
 while ($line = <$fpi>) {
 
     print STDERR $line; ## entweder: alle Zeilen ausgeben