Let tincctl use the NETNAME environment variable if no -n option is given.

This allows administrators who frequently want to work with one tinc
network to omit the -n option. Since the NETNAME variable is set by
tincd when executing scripts, this makes it slightly easier to use
tincctl from within scripts.

doc/tinc.texi

@@ -1928,6 +1928,7 @@ tincctl -n @var{netname} reload
 
 @menu
 * tincctl runtime options::
+* tincctl environment variables::
 * tincctl commands::
 * tincctl examples::
 * tincctl top::
@@ -1960,6 +1961,16 @@ Output version information and exit.
 
 @end table
 
+@c ==================================================================
+@node    tincctl environment variables
+@section tincctl environment variables
+
+@table @env
+@cindex NETNAME
+@item NETNAME
+If no netname is specified on the command line with the @option{-n} option,
+the value of this environment variable is used.
+@end table
 
 @c ==================================================================
 @node    tincctl commands

doc/tincctl.8.in

@@ -37,6 +37,13 @@ Display short list of options.
 .It Fl -version
 Output version information and exit.
 .El
+.Sh ENVIRONMENT VARIABLES
+.Bl -tag -width indent
+.It Ev NETNAME
+If no netname is specified on the command line with the
+.Fl n
+option, the value of this environment variable is used.
+.El
 .Sh COMMANDS
 .zZ
 .Bl -tag -width indent

src/tincctl.c

@@ -138,6 +138,12 @@ static bool parse_options(int argc, char **argv) {
 		}
 	}
 
+	if(!netname) {
+		netname = getenv("NETNAME");
+		if(netname)
+			netname = xstrdup(netname);
+	}
+
 	return true;
 }