ns2: Add VTY option to change the max write queue size for UDP

Ensure that existing binds are updated as well.
In some cases the default max length of the osmo_io txqueue could be too
small. Set it to 128 by default and add a VTY option to change it for
an NSI (program-wide).

Change-Id: I993b87fd6b83b3981f5e293f70b931075afec715
Related: SYS#6550
diff --git a/src/gb/gprs_ns2.c b/src/gb/gprs_ns2.c
index 02d2266..4e496c1 100644
--- a/src/gb/gprs_ns2.c
+++ b/src/gb/gprs_ns2.c
@@ -1451,6 +1451,8 @@
 	nsi->timeout[NS_TOUT_TSNS_CONFIG_RETRIES] = 3;
 	nsi->timeout[NS_TOUT_TSNS_PROCEDURES_RETRIES] = 3;
 
+	nsi->txqueue_max_length = NS_DEFAULT_TXQUEUE_MAX_LENGTH;
+
 	return nsi;
 }