Change wired-hello-interval default to 4 seconds.

Make the intervals the same on wired and wireless links.  The previous
defaults worked well in practice, but people found the discrepancy confusing.

babeld.c

@@ -257,7 +257,7 @@ main(int argc, char **argv)
     wireless_hello_interval = MAX(wireless_hello_interval, 5);
 
     if(wired_hello_interval <= 0)
-        wired_hello_interval = 20000;
+        wired_hello_interval = 4000;
     wired_hello_interval = MAX(wired_hello_interval, 5);
 
     if(parasitic && allow_duplicates >= 0) {

babeld.man

@@ -42,7 +42,7 @@ sent on wireless interfaces.  The default is 4 seconds.
 .TP
 .BI \-H " wired-hello-interval"
 Specify the interval in seconds at which scheduled hello packets are
-sent on wired interfaces.  The default is 20 seconds.
+sent on wired interfaces.  The default is 4 seconds.
 .TP
 .BI \-i " idle-hello-interval"
 Enable detection of idle networks (networks on which we haven't received